• LVDS line receiver with an unencrypted SPICE model?

    From Phil Hobbs@21:1/5 to All on Wed Jul 3 21:49:10 2024
    So I have this new gig making a very low cost TDR system for soil
    moisture and conductivity, for use in agriculture. All very interesting
    and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to
    generate the TX pulse and the sampling gate. One will have a fixed
    comparison voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs performance tradeoff
    we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002.
    It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and
    and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE
    things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere
    mortals are allowed to use?

    Thanks

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From boB@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Thu Jul 4 14:00:46 2024
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system for soil
    moisture and conductivity, for use in agriculture. All very interesting
    and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to
    generate the TX pulse and the sampling gate. One will have a fixed >comparison voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs performance tradeoff
    we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002.
    It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and
    and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE >things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere
    mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so that a model
    could be made ? Block diagram or something ?

    boB

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to boB on Thu Jul 4 21:10:20 2024
    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system for soil
    moisture and conductivity, for use in agriculture. All very interesting
    and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to
    generate the TX pulse and the sampling gate. One will have a fixed
    comparison voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs performance tradeoff
    we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002.
    It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and
    and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE
    things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere
    mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so that a model
    could be made ? Block diagram or something ?

    boB



    Too much like work—it’ll be much easier to build the circuit and measure it. I can simulate most of the fast stuff, because it’s all discrete.

    Thanks

    Phil Hobbs

    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From john larkin @21:1/5 to pcdhSpamMeSenseless@electrooptical. on Thu Jul 4 15:23:46 2024
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system for soil
    moisture and conductivity, for use in agriculture. All very interesting >>> and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to
    generate the TX pulse and the sampling gate. One will have a fixed
    comparison voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs performance tradeoff
    we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002.
    It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and
    and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE
    things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere
    mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so that a model
    could be made ? Block diagram or something ?

    boB



    Too much like work—it’ll be much easier to build the circuit and measure
    it. I can simulate most of the fast stuff, because it’s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS
    receivers and all seem to work like decent RRIO comparators. You can
    put an RC ramp into one input and a DAC into the other and make a
    ps-resolution programmable delay.

    I've given up on fast linear ramps... too much work. An RC with a bit
    of polynomial DAC correction works great.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to All on Thu Jul 4 23:12:43 2024
    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system for soil
    moisture and conductivity, for use in agriculture. All very interesting >>>> and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to
    generate the TX pulse and the sampling gate. One will have a fixed
    comparison voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs performance tradeoff >>>> we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002.
    It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and >>>> and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE >>>> things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere
    mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so that a model
    could be made ? Block diagram or something ?

    boB



    Too much like work—it’ll be much easier to build the circuit and measure >> it. I can simulate most of the fast stuff, because it’s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS
    receivers and all seem to work like decent RRIO comparators. You can
    put an RC ramp into one input and a DAC into the other and make a ps-resolution programmable delay.

    I've given up on fast linear ramps... too much work. An RC with a bit
    of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge, probably a
    BFP740. It won’t have much chance to oscillate, so it should be fine.

    You have to be a bit careful, because the prop delay depends some on the CM voltage. It’s a bit like the offset voltage of a RRIO op amp—squirrely things happen within a couple of V_BEs of one rail.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to All on Thu Jul 4 19:35:11 2024
    On 2024-07-04 18:58, JM wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system for soil
    moisture and conductivity, for use in agriculture. All very interesting
    and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to
    generate the TX pulse and the sampling gate. One will have a fixed
    comparison voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs performance tradeoff
    we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002.
    It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and
    and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE
    things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere
    mortals are allowed to use?


    I have sent you a link.

    Thanks, John! We'll see if LTspice can digest it.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From john larkin @21:1/5 to pcdhSpamMeSenseless@electrooptical. on Thu Jul 4 20:24:38 2024
    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system for soil
    moisture and conductivity, for use in agriculture. All very interesting >>>>> and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to
    generate the TX pulse and the sampling gate. One will have a fixed
    comparison voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs performance tradeoff >>>>> we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002. >>>>> It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and >>>>> and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE >>>>> things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere
    mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so that a model
    could be made ? Block diagram or something ?

    boB



    Too much like work?it?ll be much easier to build the circuit and measure >>> it. I can simulate most of the fast stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS
    receivers and all seem to work like decent RRIO comparators. You can
    put an RC ramp into one input and a DAC into the other and make a
    ps-resolution programmable delay.

    I've given up on fast linear ramps... too much work. An RC with a bit
    of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge, probably a
    BFP740. It won’t have much chance to oscillate, so it should be fine.

    You have to be a bit careful, because the prop delay depends some on the CM >voltage. It’s a bit like the offset voltage of a RRIO op amp—squirrely
    things happen within a couple of V_BEs of one rail.

    Cheers

    Phil Hobbs

    The LVDS receivers that we use do behave much better when the inputs
    are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay times,
    and calculate a polynomial. Ramp curvature and cmrr don't matter much
    anymore.

    Take a look at the SY88022 laser driver chip. It's stunning. Diff in,
    brutal but clean 25 ps outputs, smooth amplitude control down to zero,
    about $7.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Phil Hobbs on Fri Jul 5 16:02:56 2024
    Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
    On 2024-07-04 18:58, JM wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system for soil
    moisture and conductivity, for use in agriculture. All very interesting >>> and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to
    generate the TX pulse and the sampling gate. One will have a fixed
    comparison voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs performance tradeoff
    we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002.
    It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and
    and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE
    things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere
    mortals are allowed to use?


    I have sent you a link.

    Thanks, John! We'll see if LTspice can digest it.


    Ah well, not unexpectedly LTspice puked its guts over the model syntax and
    the unsupported BSIM3 version 3.11.

    I tried a few things, including changing it to call out BSIM 3.3, which
    LTspice claims to support, but no joy.

    Back to behavioral. :(

    Thanks again!

    Phil Hobbs

    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From john larkin @21:1/5 to All on Fri Jul 5 09:54:36 2024
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs ><pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system for soil
    moisture and conductivity, for use in agriculture. All very interesting >>>>>> and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to >>>>>> generate the TX pulse and the sampling gate. One will have a fixed >>>>>> comparison voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs performance tradeoff >>>>>> we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002. >>>>>> It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and >>>>>> and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE >>>>>> things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere >>>>>> mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so that a model
    could be made ? Block diagram or something ?

    boB



    Too much like work?it?ll be much easier to build the circuit and measure >>>> it. I can simulate most of the fast stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS
    receivers and all seem to work like decent RRIO comparators. You can
    put an RC ramp into one input and a DAC into the other and make a
    ps-resolution programmable delay.

    I've given up on fast linear ramps... too much work. An RC with a bit
    of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge, probably a >>BFP740. It won’t have much chance to oscillate, so it should be fine.

    You have to be a bit careful, because the prop delay depends some on the CM >>voltage. It’s a bit like the offset voltage of a RRIO op amp—squirrely >>things happen within a couple of V_BEs of one rail.

    Cheers

    Phil Hobbs

    The LVDS receivers that we use do behave much better when the inputs
    are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay times,
    and calculate a polynomial. Ramp curvature and cmrr don't matter much >anymore.

    Take a look at the SY88022 laser driver chip. It's stunning. Diff in,
    brutal but clean 25 ps outputs, smooth amplitude control down to zero,
    about $7.


    Or slam one or two SAV541 types with the FIN output. They go from off
    to about 2 ohms in a few tenths of a volt of gate drive. The newer
    packages should have less wirebond inductance than the originals.

    Package inductance probably dominates actual fast switching behavior.
    Spice models usually ignore that. I suppose that inductances could be
    inferred from s-params, but I couldn't do that.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to john larkin on Fri Jul 5 15:52:20 2024
    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system
    for soil moisture and conductivity, for use in
    agriculture. All very interesting and topical, what with
    droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two
    comparators to generate the TX pulse and the sampling
    gate. One will have a fixed comparison voltage, and the
    other one's will be set by a DAC, or possibly by a slower
    ramp, depending on the BOM vs performance tradeoff we
    wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver,
    the FIN1002. It has nice 250-300 ps edges and pretty low
    jitter, comes in SOT23, and and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty
    encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model
    that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so that
    a model could be made ? Block diagram or something ?

    boB



    Too much like work?it?ll be much easier to build the circuit
    and measure it. I can simulate most of the fast stuff,
    because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar
    LVDS receivers and all seem to work like decent RRIO
    comparators. You can put an RC ramp into one input and a DAC
    into the other and make a ps-resolution programmable delay.

    I've given up on fast linear ramps... too much work. An RC with
    a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge,
    probably a BFP740. It won’t have much chance to oscillate, so it
    should be fine.

    You have to be a bit careful, because the prop delay depends some
    on the CM voltage. It’s a bit like the offset voltage of a RRIO
    op amp—squirrely things happen within a couple of V_BEs of one
    rail.


    The LVDS receivers that we use do behave much better when the
    inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay
    times, and calculate a polynomial. Ramp curvature and cmrr don't
    matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning. Diff
    in, brutal but clean 25 ps outputs, smooth amplitude control down
    to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go from
    off to about 2 ohms in a few tenths of a volt of gate drive. The
    newer packages should have less wirebond inductance than the
    originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground, you
    won't go far wrong. Leo Bodnar has gone upmarket, so I may need to
    start using them--I've used his stuff in POC protos, but not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the pad capacitance down. A good bead right at the base tames them very nicely
    at lowish drain current, but if you let the base pad capacitance get as
    large as 0.5 pF, above about 5 mA you're toast regardless of how good
    the bead is.

    The nice thing about them for this job is that they cost $0.16 rather
    than $2. (Part of the royalty conversation is BOM savings.)

    The business end of the TDR looks like a giant meat fork, with a slide
    hammer for getting it in and out of the ground. (Fortunately the
    circuit board is at the other end of a cable from the slide hammer!)
    It's got some sort of balun inside, which I hope is a bazooka-style one
    with ferrite, rather than something more complicated.

    You really only care about suppressing the current on the outside of the shield. The classic bazooka is a 1/4-wave coaxial sleeve, shorted to
    the outer conductor at the generator end, so that it looks like an open
    circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the job
    pretty well--all you really care about is that the outside shield
    current sees a high impedance, and the phase angle doesn't matter much.

    Having control over both the TX and RX means that you can ignore a lot
    of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From john larkin @21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Jul 5 13:05:58 2024
    On Fri, 5 Jul 2024 15:52:20 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system
    for soil moisture and conductivity, for use in
    agriculture. All very interesting and topical, what with
    droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two
    comparators to generate the TX pulse and the sampling
    gate. One will have a fixed comparison voltage, and the
    other one's will be set by a DAC, or possibly by a slower
    ramp, depending on the BOM vs performance tradeoff we
    wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver,
    the FIN1002. It has nice 250-300 ps edges and pretty low
    jitter, comes in SOT23, and and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty
    encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model
    that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so that
    a model could be made ? Block diagram or something ?

    boB



    Too much like work?it?ll be much easier to build the circuit
    and measure it. I can simulate most of the fast stuff,
    because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar
    LVDS receivers and all seem to work like decent RRIO
    comparators. You can put an RC ramp into one input and a DAC
    into the other and make a ps-resolution programmable delay.

    I've given up on fast linear ramps... too much work. An RC with
    a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge,
    probably a BFP740. It won’t have much chance to oscillate, so it
    should be fine.

    You have to be a bit careful, because the prop delay depends some
    on the CM voltage. It’s a bit like the offset voltage of a RRIO
    op amp—squirrely things happen within a couple of V_BEs of one
    rail.


    The LVDS receivers that we use do behave much better when the
    inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay
    times, and calculate a polynomial. Ramp curvature and cmrr don't
    matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning. Diff
    in, brutal but clean 25 ps outputs, smooth amplitude control down
    to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go from
    off to about 2 ohms in a few tenths of a volt of gate drive. The
    newer packages should have less wirebond inductance than the
    originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground, you
    won't go far wrong. Leo Bodnar has gone upmarket, so I may need to
    start using them--I've used his stuff in POC protos, but not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the pad >capacitance down. A good bead right at the base tames them very nicely
    at lowish drain current, but if you let the base pad capacitance get as
    large as 0.5 pF, above about 5 mA you're toast regardless of how good
    the bead is.

    The nice thing about them for this job is that they cost $0.16 rather
    than $2. (Part of the royalty conversation is BOM savings.)

    The business end of the TDR looks like a giant meat fork, with a slide
    hammer for getting it in and out of the ground. (Fortunately the
    circuit board is at the other end of a cable from the slide hammer!)
    It's got some sort of balun inside, which I hope is a bazooka-style one
    with ferrite, rather than something more complicated.

    You really only care about suppressing the current on the outside of the >shield. The classic bazooka is a 1/4-wave coaxial sleeve, shorted to
    the outer conductor at the generator end, so that it looks like an open >circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the job
    pretty well--all you really care about is that the outside shield
    current sees a high impedance, and the phase angle doesn't matter much.

    Having control over both the TX and RX means that you can ignore a lot
    of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and
    deconvolve a software FIR filter to beautify it. And speed it up a bit
    maybe.

    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to john larkin on Fri Jul 5 16:30:16 2024
    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20
    -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR
    system for soil moisture and conductivity, for use
    in agriculture. All very interesting and topical,
    what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two
    comparators to generate the TX pulse and the
    sampling gate. One will have a fixed comparison
    voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs
    performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line
    receiver, the FIN1002. It has nice 250-300 ps edges
    and pretty low jitter, comes in SOT23, and and costs
    30 cents on LCSC.

    It even has SPICE models, but of course they're
    nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE
    model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so
    that a model could be made ? Block diagram or
    something ?

    boB



    Too much like work?it?ll be much easier to build the
    circuit and measure it. I can simulate most of the fast
    stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar
    LVDS receivers and all seem to work like decent RRIO
    comparators. You can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable
    delay.

    I've given up on fast linear ramps... too much work. An RC
    with a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge,
    probably a BFP740. It won’t have much chance to oscillate, so
    it should be fine.

    You have to be a bit careful, because the prop delay depends
    some on the CM voltage. It’s a bit like the offset voltage of
    a RRIO op amp—squirrely things happen within a couple of
    V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the
    inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay
    times, and calculate a polynomial. Ramp curvature and cmrr
    don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning.
    Diff in, brutal but clean 25 ps outputs, smooth amplitude
    control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go
    from off to about 2 ohms in a few tenths of a volt of gate drive.
    The newer packages should have less wirebond inductance than the
    originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do
    that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground,
    you won't go far wrong. Leo Bodnar has gone upmarket, so I may
    need to start using them--I've used his stuff in POC protos, but
    not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the
    pad capacitance down. A good bead right at the base tames them
    very nicely at lowish drain current, but if you let the base pad
    capacitance get as large as 0.5 pF, above about 5 mA you're toast
    regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16
    rather than $2. (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a
    slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from
    the slide hammer!) It's got some sort of balun inside, which I hope
    is a bazooka-style one with ferrite, rather than something more
    complicated.

    You really only care about suppressing the current on the outside
    of the shield. The classic bazooka is a 1/4-wave coaxial sleeve,
    shorted to the outer conductor at the generator end, so that it
    looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the
    job pretty well--all you really care about is that the outside
    shield current sees a high impedance, and the phase angle doesn't
    matter much.

    Having control over both the TX and RX means that you can ignore a
    lot of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and
    deconvolve a software FIR filter to beautify it. And speed it up a
    bit maybe.


    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1


    Are you, of all people, suggesting that we use that nasty evil ill-posed ill-conditioned _deconvolution_ thing you used to beat me up about?

    Nah, must be my ear wax. ;)

    Cheers

    Phil Hobbs

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Phil Hobbs on Fri Jul 5 20:41:01 2024
    On 2024-07-05 16:30, Phil Hobbs wrote:
    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20
    -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR
    system for soil moisture and conductivity, for use
    in agriculture.  All very interesting and topical,
    what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two
    comparators to generate the TX pulse and the
    sampling gate.  One will have a fixed comparison
    voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs
    performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line
    receiver, the FIN1002. It has nice 250-300 ps edges
    and pretty low jitter, comes in SOT23, and and costs
    30 cents on LCSC.

    It even has SPICE models, but of course they're
    nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE
    model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so
    that a model could be made ?  Block diagram or
    something ?

    boB



    Too much like work?it?ll be much easier to build the
    circuit and measure it. I can simulate most of the fast
    stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS
    receivers and all seem to work like decent RRIO comparators. You >>>>>>> can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable
    delay.

    I've given up on fast linear ramps... too much work. An RC
    with a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge,
    probably a BFP740. It won’t have much chance to oscillate, so
    it should be fine.

    You have to be a bit careful, because the prop delay depends
    some on the CM voltage. It’s a bit like the offset voltage of
    a RRIO op amp—squirrely things happen within a couple of
    V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the
    inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay
    times, and calculate a polynomial. Ramp curvature and cmrr
    don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning.
    Diff in, brutal but clean 25 ps outputs, smooth amplitude
    control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go
    from off to about 2 ohms in a few tenths of a volt of gate drive.
    The newer packages should have less wirebond inductance than the
    originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do
    that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground,
    you won't go far wrong.  Leo Bodnar has gone upmarket, so I may
    need to start using them--I've used his stuff in POC protos, but
    not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the
    pad capacitance down.  A good bead right at the base tames them
    very nicely at lowish drain current, but if you let the base pad
    capacitance get as large as 0.5 pF,  above about 5 mA you're toast
    regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16
    rather than $2.  (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a
    slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from
    the slide hammer!) It's got some sort of balun inside, which I hope
    is a bazooka-style one with ferrite, rather than something more
    complicated.

    You really only care about suppressing the current on the outside
    of the shield.  The classic bazooka is a 1/4-wave coaxial sleeve,
    shorted to the outer conductor at the generator end, so that it
    looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the
    job pretty well--all you really care about is that the outside
    shield current sees a high impedance, and the phase angle doesn't
    matter much.

    Having control over both the TX and RX means that you can ignore a
    lot of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and
    deconvolve a software FIR filter to beautify it. And speed it up a
    bit maybe.


    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1



    Are you, of all people, suggesting that we use that nasty evil ill-posed ill-conditioned _deconvolution_ thing you used to beat me up about?

    Nah, must be my ear wax. ;)

    If I can come up with a good online cal scheme, that's potentially a
    helpful method. I really don't want to have to be Agoston Agoston. ;)

    But you can make things like blowby, nonlinear capacitance, poor
    sampling efficiency due to the diode R * sampling C time constant being
    too slow, and so on, go away.

    This is done by (1) doing the second-stage sampling when nothing is
    going on at the input. That fixes the blowby problem.

    Then (2), you feed back the second-stage sampled value to the sampling
    cap, which is the normal sampling loop approach.

    Finally (3), you take 20 samples per delay value, so that the sampling
    loop converges, resulting in zero volts across the sampling bridge at
    the sampling instant. That gets rid of the nonlinear capacitance problem.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From john larkin @21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Jul 5 17:44:26 2024
    On Fri, 5 Jul 2024 16:30:16 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20
    -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR
    system for soil moisture and conductivity, for use
    in agriculture. All very interesting and topical,
    what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two
    comparators to generate the TX pulse and the
    sampling gate. One will have a fixed comparison
    voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs
    performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line
    receiver, the FIN1002. It has nice 250-300 ps edges
    and pretty low jitter, comes in SOT23, and and costs
    30 cents on LCSC.

    It even has SPICE models, but of course they're
    nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE
    model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so
    that a model could be made ? Block diagram or
    something ?

    boB



    Too much like work?it?ll be much easier to build the
    circuit and measure it. I can simulate most of the fast
    stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar
    LVDS receivers and all seem to work like decent RRIO
    comparators. You can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable
    delay.

    I've given up on fast linear ramps... too much work. An RC
    with a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge,
    probably a BFP740. It won’t have much chance to oscillate, so
    it should be fine.

    You have to be a bit careful, because the prop delay depends
    some on the CM voltage. It’s a bit like the offset voltage of
    a RRIO op amp—squirrely things happen within a couple of
    V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the
    inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay
    times, and calculate a polynomial. Ramp curvature and cmrr
    don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning.
    Diff in, brutal but clean 25 ps outputs, smooth amplitude
    control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go
    from off to about 2 ohms in a few tenths of a volt of gate drive.
    The newer packages should have less wirebond inductance than the
    originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do
    that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground,
    you won't go far wrong. Leo Bodnar has gone upmarket, so I may
    need to start using them--I've used his stuff in POC protos, but
    not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the
    pad capacitance down. A good bead right at the base tames them
    very nicely at lowish drain current, but if you let the base pad
    capacitance get as large as 0.5 pF, above about 5 mA you're toast
    regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16
    rather than $2. (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a
    slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from
    the slide hammer!) It's got some sort of balun inside, which I hope
    is a bazooka-style one with ferrite, rather than something more
    complicated.

    You really only care about suppressing the current on the outside
    of the shield. The classic bazooka is a 1/4-wave coaxial sleeve,
    shorted to the outer conductor at the generator end, so that it
    looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the
    job pretty well--all you really care about is that the outside
    shield current sees a high impedance, and the phase angle doesn't
    matter much.

    Having control over both the TX and RX means that you can ignore a
    lot of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and
    deconvolve a software FIR filter to beautify it. And speed it up a
    bit maybe.

    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1


    Are you, of all people, suggesting that we use that nasty evil ill-posed >ill-conditioned _deconvolution_ thing you used to beat me up about?

    Nah, must be my ear wax. ;)

    Cheers

    Phil Hobbs

    Me? Beat you up? I wouldnb't dare.

    "Ill posed" means that academics don't approve. Tough, guys.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From john larkin @21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Jul 5 17:48:08 2024
    On Fri, 5 Jul 2024 20:41:01 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 16:30, Phil Hobbs wrote:
    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20
    -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR
    system for soil moisture and conductivity, for use
    in agriculture.  All very interesting and topical,
    what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two
    comparators to generate the TX pulse and the
    sampling gate.  One will have a fixed comparison
    voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs
    performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line
    receiver, the FIN1002. It has nice 250-300 ps edges
    and pretty low jitter, comes in SOT23, and and costs
    30 cents on LCSC.

    It even has SPICE models, but of course they're
    nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE
    model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so
    that a model could be made ?  Block diagram or
    something ?

    boB



    Too much like work?it?ll be much easier to build the
    circuit and measure it. I can simulate most of the fast
    stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS >>>>>>>> receivers and all seem to work like decent RRIO comparators. You >>>>>>>> can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable
    delay.

    I've given up on fast linear ramps... too much work. An RC
    with a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge,
    probably a BFP740. It won’t have much chance to oscillate, so
    it should be fine.

    You have to be a bit careful, because the prop delay depends
    some on the CM voltage. It’s a bit like the offset voltage of
    a RRIO op amp—squirrely things happen within a couple of
    V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the
    inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay
    times, and calculate a polynomial. Ramp curvature and cmrr
    don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning.
    Diff in, brutal but clean 25 ps outputs, smooth amplitude
    control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go
    from off to about 2 ohms in a few tenths of a volt of gate drive.
    The newer packages should have less wirebond inductance than the
    originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do
    that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground,
    you won't go far wrong.  Leo Bodnar has gone upmarket, so I may
    need to start using them--I've used his stuff in POC protos, but
    not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the
    pad capacitance down.  A good bead right at the base tames them
    very nicely at lowish drain current, but if you let the base pad
    capacitance get as large as 0.5 pF,  above about 5 mA you're toast
    regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16
    rather than $2.  (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a
    slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from
    the slide hammer!) It's got some sort of balun inside, which I hope
    is a bazooka-style one with ferrite, rather than something more
    complicated.

    You really only care about suppressing the current on the outside
    of the shield.  The classic bazooka is a 1/4-wave coaxial sleeve,
    shorted to the outer conductor at the generator end, so that it
    looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the
    job pretty well--all you really care about is that the outside
    shield current sees a high impedance, and the phase angle doesn't
    matter much.

    Having control over both the TX and RX means that you can ignore a
    lot of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and
    deconvolve a software FIR filter to beautify it. And speed it up a
    bit maybe.


    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1



    Are you, of all people, suggesting that we use that nasty evil ill-posed
    ill-conditioned _deconvolution_ thing you used to beat me up about?

    Nah, must be my ear wax. ;)

    If I can come up with a good online cal scheme, that's potentially a
    helpful method. I really don't want to have to be Agoston Agoston. ;)

    But you can make things like blowby, nonlinear capacitance, poor
    sampling efficiency due to the diode R * sampling C time constant being
    too slow, and so on, go away.

    This is done by (1) doing the second-stage sampling when nothing is
    going on at the input. That fixes the blowby problem.

    Then (2), you feed back the second-stage sampled value to the sampling
    cap, which is the normal sampling loop approach.

    Finally (3), you take 20 samples per delay value, so that the sampling
    loop converges, resulting in zero volts across the sampling bridge at
    the sampling instant. That gets rid of the nonlinear capacitance problem.

    Cheers

    Phil Hobbs

    What sort of rise time are you after?

    There are goofy possibilities.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to john larkin on Fri Jul 5 21:35:29 2024
    On 2024-07-05 20:48, john larkin wrote:
    On Fri, 5 Jul 2024 20:41:01 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 16:30, Phil Hobbs wrote:
    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20
    -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR
    system for soil moisture and conductivity, for use
    in agriculture.  All very interesting and topical,
    what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two
    comparators to generate the TX pulse and the
    sampling gate.  One will have a fixed comparison
    voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs
    performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line
    receiver, the FIN1002. It has nice 250-300 ps edges
    and pretty low jitter, comes in SOT23, and and costs
    30 cents on LCSC.

    It even has SPICE models, but of course they're
    nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE
    model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so
    that a model could be made ?  Block diagram or
    something ?

    boB



    Too much like work?it?ll be much easier to build the
    circuit and measure it. I can simulate most of the fast
    stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS >>>>>>>>> receivers and all seem to work like decent RRIO comparators. You >>>>>>>>> can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable
    delay.

    I've given up on fast linear ramps... too much work. An RC
    with a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge,
    probably a BFP740. It won’t have much chance to oscillate, so >>>>>>>> it should be fine.

    You have to be a bit careful, because the prop delay depends
    some on the CM voltage. It’s a bit like the offset voltage of >>>>>>>> a RRIO op amp—squirrely things happen within a couple of
    V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the
    inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay
    times, and calculate a polynomial. Ramp curvature and cmrr
    don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning.
    Diff in, brutal but clean 25 ps outputs, smooth amplitude
    control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go
    from off to about 2 ohms in a few tenths of a volt of gate drive.
    The newer packages should have less wirebond inductance than the
    originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do
    that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground,
    you won't go far wrong.  Leo Bodnar has gone upmarket, so I may
    need to start using them--I've used his stuff in POC protos, but
    not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the
    pad capacitance down.  A good bead right at the base tames them
    very nicely at lowish drain current, but if you let the base pad
    capacitance get as large as 0.5 pF,  above about 5 mA you're toast
    regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16
    rather than $2.  (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a
    slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from
    the slide hammer!) It's got some sort of balun inside, which I hope
    is a bazooka-style one with ferrite, rather than something more
    complicated.

    You really only care about suppressing the current on the outside
    of the shield.  The classic bazooka is a 1/4-wave coaxial sleeve,
    shorted to the outer conductor at the generator end, so that it
    looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the
    job pretty well--all you really care about is that the outside
    shield current sees a high impedance, and the phase angle doesn't
    matter much.

    Having control over both the TX and RX means that you can ignore a
    lot of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and
    deconvolve a software FIR filter to beautify it. And speed it up a
    bit maybe.


    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1



    Are you, of all people, suggesting that we use that nasty evil ill-posed >>> ill-conditioned _deconvolution_ thing you used to beat me up about?

    Nah, must be my ear wax. ;)

    If I can come up with a good online cal scheme, that's potentially a
    helpful method. I really don't want to have to be Agoston Agoston. ;)

    But you can make things like blowby, nonlinear capacitance, poor
    sampling efficiency due to the diode R * sampling C time constant being
    too slow, and so on, go away.

    This is done by (1) doing the second-stage sampling when nothing is
    going on at the input. That fixes the blowby problem.

    Then (2), you feed back the second-stage sampled value to the sampling
    cap, which is the normal sampling loop approach.

    Finally (3), you take 20 samples per delay value, so that the sampling
    loop converges, resulting in zero volts across the sampling bridge at
    the sampling instant. That gets rid of the nonlinear capacitance problem. >>
    Cheers

    Phil Hobbs

    What sort of rise time are you after?

    There are goofy possibilities.


    For TDRing dirt, I find it hard to believe we'll need better than 100 ps.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to sunaecoNoSpam@gmail.com on Sat Jul 6 02:24:23 2024
    JM <sunaecoNoSpam@gmail.com> wrote:
    On Fri, 5 Jul 2024 16:02:56 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
    On 2024-07-04 18:58, JM wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system for soil
    moisture and conductivity, for use in agriculture. All very interesting >>>>> and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to
    generate the TX pulse and the sampling gate. One will have a fixed
    comparison voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs performance tradeoff >>>>> we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002. >>>>> It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and >>>>> and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE >>>>> things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere
    mortals are allowed to use?


    I have sent you a link.

    Thanks, John! We'll see if LTspice can digest it.


    Ah well, not unexpectedly LTspice puked its guts over the model syntax and >> the unsupported BSIM3 version 3.11.

    I tried a few things, including changing it to call out BSIM 3.3, which
    LTspice claims to support, but no joy.

    Back to behavioral. :(

    Thanks again!

    Phil Hobbs

    Just tried it in LTspiff 24.0.11 and 17.0.36. It will run if you change value of R6 (about line 214) from 0.0 to something other than 0.0 (say 1e-12).
    (This with the subckt with package model.)

    LTspice does ignore a lot of the BSIM parameters, but you get what you pay for...





    Right you are, thanks!

    Lots of warnings, but seems to do vaguely sensible things—220 ps rise, 300
    ps fall, 1.5 ns delay.

    Overall, probably closer than I would have got with a simple behavioral.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Bill Sloman@21:1/5 to john larkin on Sat Jul 6 14:26:12 2024
    On 6/07/2024 10:44 am, john larkin wrote:
    On Fri, 5 Jul 2024 16:30:16 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20
    -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR
    system for soil moisture and conductivity, for use
    in agriculture. All very interesting and topical,
    what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two
    comparators to generate the TX pulse and the
    sampling gate. One will have a fixed comparison
    voltage, and the other one's will be set by a DAC, or
    possibly by a slower ramp, depending on the BOM vs
    performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line
    receiver, the FIN1002. It has nice 250-300 ps edges
    and pretty low jitter, comes in SOT23, and and costs
    30 cents on LCSC.

    It even has SPICE models, but of course they're
    nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE
    model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so
    that a model could be made ? Block diagram or
    something ?

    boB



    Too much like work?it?ll be much easier to build the
    circuit and measure it. I can simulate most of the fast
    stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar
    LVDS receivers and all seem to work like decent RRIO
    comparators. You can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable
    delay.

    I've given up on fast linear ramps... too much work. An RC
    with a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge,
    probably a BFP740. It won’t have much chance to oscillate, so
    it should be fine.

    You have to be a bit careful, because the prop delay depends
    some on the CM voltage. It’s a bit like the offset voltage of
    a RRIO op amp—squirrely things happen within a couple of
    V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the
    inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay
    times, and calculate a polynomial. Ramp curvature and cmrr
    don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning.
    Diff in, brutal but clean 25 ps outputs, smooth amplitude
    control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go
    from off to about 2 ohms in a few tenths of a volt of gate drive.
    The newer packages should have less wirebond inductance than the
    originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do
    that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground,
    you won't go far wrong. Leo Bodnar has gone upmarket, so I may
    need to start using them--I've used his stuff in POC protos, but
    not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the
    pad capacitance down. A good bead right at the base tames them
    very nicely at lowish drain current, but if you let the base pad
    capacitance get as large as 0.5 pF, above about 5 mA you're toast
    regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16
    rather than $2. (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a
    slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from
    the slide hammer!) It's got some sort of balun inside, which I hope
    is a bazooka-style one with ferrite, rather than something more
    complicated.

    You really only care about suppressing the current on the outside
    of the shield. The classic bazooka is a 1/4-wave coaxial sleeve,
    shorted to the outer conductor at the generator end, so that it
    looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the
    job pretty well--all you really care about is that the outside
    shield current sees a high impedance, and the phase angle doesn't
    matter much.

    Having control over both the TX and RX means that you can ignore a
    lot of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and
    deconvolve a software FIR filter to beautify it. And speed it up a
    bit maybe.


    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1


    Are you, of all people, suggesting that we use that nasty evil ill-posed
    ill-conditioned _deconvolution_ thing you used to beat me up about?

    Nah, must be my ear wax. ;)

    Cheers

    Phil Hobbs

    Me? Beat you up? I wouldnb't dare.

    "Ill posed" means that academics don't approve. Tough, guys.

    Not exactly. It means that it sometimes fail to produce a useful answer.

    --
    Bill Sloman, Sydney


    --
    This email has been checked for viruses by Norton antivirus software. www.norton.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From john larkin @21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Jul 5 21:46:21 2024
    On Fri, 5 Jul 2024 21:35:29 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 20:48, john larkin wrote:
    On Fri, 5 Jul 2024 20:41:01 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 16:30, Phil Hobbs wrote:
    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20
    -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR
    system for soil moisture and conductivity, for use
    in agriculture.  All very interesting and topical,
    what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two >>>>>>>>>>>>> comparators to generate the TX pulse and the
    sampling gate.  One will have a fixed comparison
    voltage, and the other one's will be set by a DAC, or >>>>>>>>>>>>> possibly by a slower ramp, depending on the BOM vs
    performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line
    receiver, the FIN1002. It has nice 250-300 ps edges
    and pretty low jitter, comes in SOT23, and and costs >>>>>>>>>>>>> 30 cents on LCSC.

    It even has SPICE models, but of course they're
    nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE
    model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so >>>>>>>>>>>> that a model could be made ?  Block diagram or
    something ?

    boB



    Too much like work?it?ll be much easier to build the
    circuit and measure it. I can simulate most of the fast
    stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS >>>>>>>>>> receivers and all seem to work like decent RRIO comparators. You >>>>>>>>>> can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable
    delay.

    I've given up on fast linear ramps... too much work. An RC >>>>>>>>>> with a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge,
    probably a BFP740. It won’t have much chance to oscillate, so >>>>>>>>> it should be fine.

    You have to be a bit careful, because the prop delay depends >>>>>>>>> some on the CM voltage. It’s a bit like the offset voltage of >>>>>>>>> a RRIO op amp—squirrely things happen within a couple of
    V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the
    inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay >>>>>>>> times, and calculate a polynomial. Ramp curvature and cmrr
    don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning.
    Diff in, brutal but clean 25 ps outputs, smooth amplitude
    control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go
    from off to about 2 ohms in a few tenths of a volt of gate drive. >>>>>>> The newer packages should have less wirebond inductance than the >>>>>>> originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do
    that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground,
    you won't go far wrong.  Leo Bodnar has gone upmarket, so I may
    need to start using them--I've used his stuff in POC protos, but
    not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the >>>>>> pad capacitance down.  A good bead right at the base tames them
    very nicely at lowish drain current, but if you let the base pad
    capacitance get as large as 0.5 pF,  above about 5 mA you're toast >>>>>> regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16
    rather than $2.  (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a
    slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from >>>>>> the slide hammer!) It's got some sort of balun inside, which I hope >>>>>> is a bazooka-style one with ferrite, rather than something more
    complicated.

    You really only care about suppressing the current on the outside
    of the shield.  The classic bazooka is a 1/4-wave coaxial sleeve,
    shorted to the outer conductor at the generator end, so that it
    looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the >>>>>> job pretty well--all you really care about is that the outside
    shield current sees a high impedance, and the phase angle doesn't
    matter much.

    Having control over both the TX and RX means that you can ignore a >>>>>> lot of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and
    deconvolve a software FIR filter to beautify it. And speed it up a
    bit maybe.


    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1



    Are you, of all people, suggesting that we use that nasty evil ill-posed >>>> ill-conditioned _deconvolution_ thing you used to beat me up about?

    Nah, must be my ear wax. ;)

    If I can come up with a good online cal scheme, that's potentially a
    helpful method. I really don't want to have to be Agoston Agoston. ;)

    But you can make things like blowby, nonlinear capacitance, poor
    sampling efficiency due to the diode R * sampling C time constant being
    too slow, and so on, go away.

    This is done by (1) doing the second-stage sampling when nothing is
    going on at the input. That fixes the blowby problem.

    Then (2), you feed back the second-stage sampled value to the sampling
    cap, which is the normal sampling loop approach.

    Finally (3), you take 20 samples per delay value, so that the sampling
    loop converges, resulting in zero volts across the sampling bridge at
    the sampling instant. That gets rid of the nonlinear capacitance problem. >>>
    Cheers

    Phil Hobbs

    What sort of rise time are you after?

    There are goofy possibilities.


    For TDRing dirt, I find it hard to believe we'll need better than 100 ps.

    Cheers

    Phil Hobbs

    Something sort of like this might work.

    https://www.dropbox.com/scl/fi/yq7an2udjygxrd2bf0rdl/DirtLine_TDR.jpg?rlkey=jyu9gb74y6dciml307jpad8bw&raw=1

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to john larkin on Sat Jul 6 11:56:08 2024
    On 2024-07-06 00:46, john larkin wrote:
    On Fri, 5 Jul 2024 21:35:29 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 20:48, john larkin wrote:
    On Fri, 5 Jul 2024 20:41:01 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 16:30, Phil Hobbs wrote:
    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20 >>>>> -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR
    system for soil moisture and conductivity, for use >>>>>>>>>>>>>> in agriculture.  All very interesting and topical, >>>>>>>>>>>>>> what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two >>>>>>>>>>>>>> comparators to generate the TX pulse and the
    sampling gate.  One will have a fixed comparison
    voltage, and the other one's will be set by a DAC, or >>>>>>>>>>>>>> possibly by a slower ramp, depending on the BOM vs >>>>>>>>>>>>>> performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line
    receiver, the FIN1002. It has nice 250-300 ps edges >>>>>>>>>>>>>> and pretty low jitter, comes in SOT23, and and costs >>>>>>>>>>>>>> 30 cents on LCSC.

    It even has SPICE models, but of course they're
    nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE >>>>>>>>>>>>>> model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so >>>>>>>>>>>>> that a model could be made ?  Block diagram or
    something ?

    boB



    Too much like work?it?ll be much easier to build the
    circuit and measure it. I can simulate most of the fast >>>>>>>>>>>> stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS >>>>>>>>>>> receivers and all seem to work like decent RRIO comparators. You >>>>>>>>>>> can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable >>>>>>>>>>> delay.

    I've given up on fast linear ramps... too much work. An RC >>>>>>>>>>> with a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge, >>>>>>>>>> probably a BFP740. It won’t have much chance to oscillate, so >>>>>>>>>> it should be fine.

    You have to be a bit careful, because the prop delay depends >>>>>>>>>> some on the CM voltage. It’s a bit like the offset voltage of >>>>>>>>>> a RRIO op amp—squirrely things happen within a couple of >>>>>>>>>> V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the >>>>>>>>> inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay >>>>>>>>> times, and calculate a polynomial. Ramp curvature and cmrr
    don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning. >>>>>>>>> Diff in, brutal but clean 25 ps outputs, smooth amplitude
    control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go
    from off to about 2 ohms in a few tenths of a volt of gate drive. >>>>>>>> The newer packages should have less wirebond inductance than the >>>>>>>> originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do >>>>>>>> that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground, >>>>>>> you won't go far wrong.  Leo Bodnar has gone upmarket, so I may >>>>>>> need to start using them--I've used his stuff in POC protos, but >>>>>>> not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the >>>>>>> pad capacitance down.  A good bead right at the base tames them >>>>>>> very nicely at lowish drain current, but if you let the base pad >>>>>>> capacitance get as large as 0.5 pF,  above about 5 mA you're toast >>>>>>> regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16
    rather than $2.  (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a >>>>>>> slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from >>>>>>> the slide hammer!) It's got some sort of balun inside, which I hope >>>>>>> is a bazooka-style one with ferrite, rather than something more
    complicated.

    You really only care about suppressing the current on the outside >>>>>>> of the shield.  The classic bazooka is a 1/4-wave coaxial sleeve, >>>>>>> shorted to the outer conductor at the generator end, so that it
    looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the >>>>>>> job pretty well--all you really care about is that the outside
    shield current sees a high impedance, and the phase angle doesn't >>>>>>> matter much.

    Having control over both the TX and RX means that you can ignore a >>>>>>> lot of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and
    deconvolve a software FIR filter to beautify it. And speed it up a >>>>>> bit maybe.


    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1



    Are you, of all people, suggesting that we use that nasty evil ill-posed >>>>> ill-conditioned _deconvolution_ thing you used to beat me up about?

    Nah, must be my ear wax. ;)

    If I can come up with a good online cal scheme, that's potentially a
    helpful method. I really don't want to have to be Agoston Agoston. ;) >>>>
    But you can make things like blowby, nonlinear capacitance, poor
    sampling efficiency due to the diode R * sampling C time constant being >>>> too slow, and so on, go away.

    This is done by (1) doing the second-stage sampling when nothing is
    going on at the input. That fixes the blowby problem.

    Then (2), you feed back the second-stage sampled value to the sampling >>>> cap, which is the normal sampling loop approach.

    Finally (3), you take 20 samples per delay value, so that the sampling >>>> loop converges, resulting in zero volts across the sampling bridge at
    the sampling instant. That gets rid of the nonlinear capacitance problem. >>>>
    Cheers

    Phil Hobbs

    What sort of rise time are you after?

    There are goofy possibilities.


    For TDRing dirt, I find it hard to believe we'll need better than 100 ps.


    Something sort of like this might work.

    https://www.dropbox.com/scl/fi/yq7an2udjygxrd2bf0rdl/DirtLine_TDR.jpg?rlkey=jyu9gb74y6dciml307jpad8bw&raw=1


    Fun. I like it in general--it should have a nice clean Tx edge, for one
    thing, and the ON resistance of the pHEMT is a lot better than a
    Schottky diode for the same capacitance. That might eliminate the need
    to use multiple sampling pulses to get the sampling loop to converge.
    (Which would be good, because you can't easily stack samples with a
    track/hold architecture.)

    ESD protection is one possible issue, and the hold capacitor loads down
    the line throughout the pulse and return, so it has to be very small.

    So far I'm envisioning a two-diode series sampler, with a fast sampling
    pulse made by taking a fast edge from a BFP840 and differentiating it
    with a shorted Tx line, maybe with a bit of capacitive help.

    As long as the final voltages across the diodes don't change from sample
    to sample, the nonlinear capacitance funnies pretty well go away. There
    might be some schmutz due to reflections off the bridge in the OFF state
    and other discontinuities rattling round. It'll be hard to get rid of
    the reflection from where the tines hit the soil surface, for instance.

    As I say, fun.

    Cheers

    Phil Hobbs


    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From john larkin @21:1/5 to pcdhSpamMeSenseless@electrooptical. on Sat Jul 6 12:03:33 2024
    On Sat, 6 Jul 2024 11:56:08 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-06 00:46, john larkin wrote:
    On Fri, 5 Jul 2024 21:35:29 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 20:48, john larkin wrote:
    On Fri, 5 Jul 2024 20:41:01 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 16:30, Phil Hobbs wrote:
    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20 >>>>>> -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR >>>>>>>>>>>>>>> system for soil moisture and conductivity, for use >>>>>>>>>>>>>>> in agriculture.  All very interesting and topical, >>>>>>>>>>>>>>> what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two >>>>>>>>>>>>>>> comparators to generate the TX pulse and the
    sampling gate.  One will have a fixed comparison >>>>>>>>>>>>>>> voltage, and the other one's will be set by a DAC, or >>>>>>>>>>>>>>> possibly by a slower ramp, depending on the BOM vs >>>>>>>>>>>>>>> performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line
    receiver, the FIN1002. It has nice 250-300 ps edges >>>>>>>>>>>>>>> and pretty low jitter, comes in SOT23, and and costs >>>>>>>>>>>>>>> 30 cents on LCSC.

    It even has SPICE models, but of course they're
    nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE >>>>>>>>>>>>>>> model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so >>>>>>>>>>>>>> that a model could be made ?  Block diagram or
    something ?

    boB



    Too much like work?it?ll be much easier to build the >>>>>>>>>>>>> circuit and measure it. I can simulate most of the fast >>>>>>>>>>>>> stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS >>>>>>>>>>>> receivers and all seem to work like decent RRIO comparators. You >>>>>>>>>>>> can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable >>>>>>>>>>>> delay.

    I've given up on fast linear ramps... too much work. An RC >>>>>>>>>>>> with a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge, >>>>>>>>>>> probably a BFP740. It won’t have much chance to oscillate, so >>>>>>>>>>> it should be fine.

    You have to be a bit careful, because the prop delay depends >>>>>>>>>>> some on the CM voltage. It’s a bit like the offset voltage of >>>>>>>>>>> a RRIO op amp—squirrely things happen within a couple of >>>>>>>>>>> V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the >>>>>>>>>> inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay >>>>>>>>>> times, and calculate a polynomial. Ramp curvature and cmrr >>>>>>>>>> don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning. >>>>>>>>>> Diff in, brutal but clean 25 ps outputs, smooth amplitude
    control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go >>>>>>>>> from off to about 2 ohms in a few tenths of a volt of gate drive. >>>>>>>>> The newer packages should have less wirebond inductance than the >>>>>>>>> originals.

    Package inductance probably dominates actual fast switching
    behavior. Spice models usually ignore that. I suppose that
    inductances could be inferred from s-params, but I couldn't do >>>>>>>>> that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground, >>>>>>>> you won't go far wrong.  Leo Bodnar has gone upmarket, so I may >>>>>>>> need to start using them--I've used his stuff in POC protos, but >>>>>>>> not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the >>>>>>>> pad capacitance down.  A good bead right at the base tames them >>>>>>>> very nicely at lowish drain current, but if you let the base pad >>>>>>>> capacitance get as large as 0.5 pF,  above about 5 mA you're toast >>>>>>>> regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16 >>>>>>>> rather than $2.  (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a >>>>>>>> slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from >>>>>>>> the slide hammer!) It's got some sort of balun inside, which I hope >>>>>>>> is a bazooka-style one with ferrite, rather than something more >>>>>>>> complicated.

    You really only care about suppressing the current on the outside >>>>>>>> of the shield.  The classic bazooka is a 1/4-wave coaxial sleeve, >>>>>>>> shorted to the outer conductor at the generator end, so that it >>>>>>>> looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the >>>>>>>> job pretty well--all you really care about is that the outside >>>>>>>> shield current sees a high impedance, and the phase angle doesn't >>>>>>>> matter much.

    Having control over both the TX and RX means that you can ignore a >>>>>>>> lot of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and >>>>>>> deconvolve a software FIR filter to beautify it. And speed it up a >>>>>>> bit maybe.


    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1



    Are you, of all people, suggesting that we use that nasty evil ill-posed >>>>>> ill-conditioned _deconvolution_ thing you used to beat me up about? >>>>>>
    Nah, must be my ear wax. ;)

    If I can come up with a good online cal scheme, that's potentially a >>>>> helpful method. I really don't want to have to be Agoston Agoston. ;) >>>>>
    But you can make things like blowby, nonlinear capacitance, poor
    sampling efficiency due to the diode R * sampling C time constant being >>>>> too slow, and so on, go away.

    This is done by (1) doing the second-stage sampling when nothing is
    going on at the input. That fixes the blowby problem.

    Then (2), you feed back the second-stage sampled value to the sampling >>>>> cap, which is the normal sampling loop approach.

    Finally (3), you take 20 samples per delay value, so that the sampling >>>>> loop converges, resulting in zero volts across the sampling bridge at >>>>> the sampling instant. That gets rid of the nonlinear capacitance problem.

    Cheers

    Phil Hobbs

    What sort of rise time are you after?

    There are goofy possibilities.


    For TDRing dirt, I find it hard to believe we'll need better than 100 ps.


    Something sort of like this might work.

    https://www.dropbox.com/scl/fi/yq7an2udjygxrd2bf0rdl/DirtLine_TDR.jpg?rlkey=jyu9gb74y6dciml307jpad8bw&raw=1


    Fun. I like it in general--it should have a nice clean Tx edge, for one >thing, and the ON resistance of the pHEMT is a lot better than a
    Schottky diode for the same capacitance. That might eliminate the need
    to use multiple sampling pulses to get the sampling loop to converge.
    (Which would be good, because you can't easily stack samples with a >track/hold architecture.)

    ESD protection is one possible issue, and the hold capacitor loads down
    the line throughout the pulse and return, so it has to be very small.

    So far I'm envisioning a two-diode series sampler, with a fast sampling
    pulse made by taking a fast edge from a BFP840 and differentiating it
    with a shorted Tx line, maybe with a bit of capacitive help.

    As long as the final voltages across the diodes don't change from sample
    to sample, the nonlinear capacitance funnies pretty well go away. There >might be some schmutz due to reflections off the bridge in the OFF state
    and other discontinuities rattling round. It'll be hard to get rid of
    the reflection from where the tines hit the soil surface, for instance.

    As I say, fun.

    Cheers

    Phil Hobbs

    How about this?

    https://www.dropbox.com/scl/fi/lo60weugmmmzkg1w5aykd/Dirt_Line_2.jpg?rlkey=96nrdt0deouoxa85sv2bogdqx&raw=1

    It has some nice behavior around the diode capacitance feed-thru. At
    sample time T2, nodes D and L both slam to ground. Before then, they
    track.

    This slow sampler drives C1 hard, unlike a faster feedback sampler
    that can only poke a bit of incremental charge into the cap each shot.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to All on Sat Jul 6 18:09:57 2024
    On 2024-07-05 23:50, JM wrote:
    On Sat, 6 Jul 2024 02:24:23 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    JM <sunaecoNoSpam@gmail.com> wrote:
    On Fri, 5 Jul 2024 16:02:56 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
    On 2024-07-04 18:58, JM wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR system for soil >>>>>>> moisture and conductivity, for use in agriculture. All very interesting
    and topical, what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two comparators to >>>>>>> generate the TX pulse and the sampling gate. One will have a fixed >>>>>>> comparison voltage, and the other one's will be set by a DAC, or >>>>>>> possibly by a slower ramp, depending on the BOM vs performance tradeoff >>>>>>> we wind up with.

    Sooo, naturally I pulled out my fave LVDS line receiver, the FIN1002. >>>>>>> It has nice 250-300 ps edges and pretty low jitter, comes in SOT23, and >>>>>>> and costs 30 cents on LCSC.

    It even has SPICE models, but of course they're nasty encrypted HSPICE >>>>>>> things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE model that mere >>>>>>> mortals are allowed to use?


    I have sent you a link.

    Thanks, John! We'll see if LTspice can digest it.


    Ah well, not unexpectedly LTspice puked its guts over the model syntax and >>>> the unsupported BSIM3 version 3.11.

    I tried a few things, including changing it to call out BSIM 3.3, which >>>> LTspice claims to support, but no joy.

    Back to behavioral. :(

    Thanks again!

    Phil Hobbs

    Just tried it in LTspiff 24.0.11 and 17.0.36. It will run if you change >>> value of R6 (about line 214) from 0.0 to something other than 0.0 (say 1e-12).
    (This with the subckt with package model.)

    LTspice does ignore a lot of the BSIM parameters, but you get what you pay for...





    Right you are, thanks!

    Lots of warnings, but seems to do vaguely sensible things—220 ps rise, 300 >> ps fall, 1.5 ns delay.

    Overall, probably closer than I would have got with a simple behavioral.

    Cheers

    Phil Hobbs

    Here's an LTspud encrypted model if anyone wants it.

    https://1drv.ms/u/c/1af24d72a509cd48/EWvKKE3EU2ZHsl22wxBKsaoBJOYfUEwW7nFYEI7TUoJpkQ?e=rQHVNW

    Filed away. Good to have one that I can share with the customers.

    Thanks again!

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to john larkin on Tue Jul 9 16:45:07 2024
    On 2024-07-06 15:03, john larkin wrote:
    On Sat, 6 Jul 2024 11:56:08 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-06 00:46, john larkin wrote:
    On Fri, 5 Jul 2024 21:35:29 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 20:48, john larkin wrote:
    On Fri, 5 Jul 2024 20:41:01 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 16:30, Phil Hobbs wrote:
    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20 >>>>>>> -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I have this new gig making a very low cost TDR >>>>>>>>>>>>>>>> system for soil moisture and conductivity, for use >>>>>>>>>>>>>>>> in agriculture.  All very interesting and topical, >>>>>>>>>>>>>>>> what with droughts and low aquifers and all.

    As one does, I'm planning to use a fast ramp and two >>>>>>>>>>>>>>>> comparators to generate the TX pulse and the
    sampling gate.  One will have a fixed comparison >>>>>>>>>>>>>>>> voltage, and the other one's will be set by a DAC, or >>>>>>>>>>>>>>>> possibly by a slower ramp, depending on the BOM vs >>>>>>>>>>>>>>>> performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line >>>>>>>>>>>>>>>> receiver, the FIN1002. It has nice 250-300 ps edges >>>>>>>>>>>>>>>> and pretty low jitter, comes in SOT23, and and costs >>>>>>>>>>>>>>>> 30 cents on LCSC.

    It even has SPICE models, but of course they're >>>>>>>>>>>>>>>> nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE >>>>>>>>>>>>>>>> model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so >>>>>>>>>>>>>>> that a model could be made ?  Block diagram or
    something ?

    boB



    Too much like work?it?ll be much easier to build the >>>>>>>>>>>>>> circuit and measure it. I can simulate most of the fast >>>>>>>>>>>>>> stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS >>>>>>>>>>>>> receivers and all seem to work like decent RRIO comparators. You >>>>>>>>>>>>> can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable >>>>>>>>>>>>> delay.

    I've given up on fast linear ramps... too much work. An RC >>>>>>>>>>>>> with a bit of polynomial DAC correction works great.

    I’m planning to use some fast gain to sharpen up the edge, >>>>>>>>>>>> probably a BFP740. It won’t have much chance to oscillate, so >>>>>>>>>>>> it should be fine.

    You have to be a bit careful, because the prop delay depends >>>>>>>>>>>> some on the CM voltage. It’s a bit like the offset voltage of >>>>>>>>>>>> a RRIO op amp—squirrely things happen within a couple of >>>>>>>>>>>> V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the >>>>>>>>>>> inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay >>>>>>>>>>> times, and calculate a polynomial. Ramp curvature and cmrr >>>>>>>>>>> don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning. >>>>>>>>>>> Diff in, brutal but clean 25 ps outputs, smooth amplitude >>>>>>>>>>> control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go >>>>>>>>>> from off to about 2 ohms in a few tenths of a volt of gate drive. >>>>>>>>>> The newer packages should have less wirebond inductance than the >>>>>>>>>> originals.

    Package inductance probably dominates actual fast switching >>>>>>>>>> behavior. Spice models usually ignore that. I suppose that >>>>>>>>>> inductances could be inferred from s-params, but I couldn't do >>>>>>>>>> that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground, >>>>>>>>> you won't go far wrong.  Leo Bodnar has gone upmarket, so I may >>>>>>>>> need to start using them--I've used his stuff in POC protos, but >>>>>>>>> not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the >>>>>>>>> pad capacitance down.  A good bead right at the base tames them >>>>>>>>> very nicely at lowish drain current, but if you let the base pad >>>>>>>>> capacitance get as large as 0.5 pF,  above about 5 mA you're toast >>>>>>>>> regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16 >>>>>>>>> rather than $2.  (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a >>>>>>>>> slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from >>>>>>>>> the slide hammer!) It's got some sort of balun inside, which I hope >>>>>>>>> is a bazooka-style one with ferrite, rather than something more >>>>>>>>> complicated.

    You really only care about suppressing the current on the outside >>>>>>>>> of the shield.  The classic bazooka is a 1/4-wave coaxial sleeve, >>>>>>>>> shorted to the outer conductor at the generator end, so that it >>>>>>>>> looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the >>>>>>>>> job pretty well--all you really care about is that the outside >>>>>>>>> shield current sees a high impedance, and the phase angle doesn't >>>>>>>>> matter much.

    Having control over both the TX and RX means that you can ignore a >>>>>>>>> lot of stuff that real sampler designs have to worry about.

    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and >>>>>>>> deconvolve a software FIR filter to beautify it. And speed it up a >>>>>>>> bit maybe.


    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1



    Are you, of all people, suggesting that we use that nasty evil ill-posed
    ill-conditioned _deconvolution_ thing you used to beat me up about? >>>>>>>
    Nah, must be my ear wax. ;)

    If I can come up with a good online cal scheme, that's potentially a >>>>>> helpful method. I really don't want to have to be Agoston Agoston. ;) >>>>>>
    But you can make things like blowby, nonlinear capacitance, poor
    sampling efficiency due to the diode R * sampling C time constant being >>>>>> too slow, and so on, go away.

    This is done by (1) doing the second-stage sampling when nothing is >>>>>> going on at the input. That fixes the blowby problem.

    Then (2), you feed back the second-stage sampled value to the sampling >>>>>> cap, which is the normal sampling loop approach.

    Finally (3), you take 20 samples per delay value, so that the sampling >>>>>> loop converges, resulting in zero volts across the sampling bridge at >>>>>> the sampling instant. That gets rid of the nonlinear capacitance problem.

    Cheers

    Phil Hobbs

    What sort of rise time are you after?

    There are goofy possibilities.


    For TDRing dirt, I find it hard to believe we'll need better than 100 ps. >>

    Something sort of like this might work.

    https://www.dropbox.com/scl/fi/yq7an2udjygxrd2bf0rdl/DirtLine_TDR.jpg?rlkey=jyu9gb74y6dciml307jpad8bw&raw=1


    Fun. I like it in general--it should have a nice clean Tx edge, for one
    thing, and the ON resistance of the pHEMT is a lot better than a
    Schottky diode for the same capacitance. That might eliminate the need
    to use multiple sampling pulses to get the sampling loop to converge.
    (Which would be good, because you can't easily stack samples with a
    track/hold architecture.)

    ESD protection is one possible issue, and the hold capacitor loads down
    the line throughout the pulse and return, so it has to be very small.

    So far I'm envisioning a two-diode series sampler, with a fast sampling
    pulse made by taking a fast edge from a BFP840 and differentiating it
    with a shorted Tx line, maybe with a bit of capacitive help.

    As long as the final voltages across the diodes don't change from sample
    to sample, the nonlinear capacitance funnies pretty well go away. There
    might be some schmutz due to reflections off the bridge in the OFF state
    and other discontinuities rattling round. It'll be hard to get rid of
    the reflection from where the tines hit the soil surface, for instance.

    As I say, fun.

    Cheers

    Phil Hobbs

    How about this?

    https://www.dropbox.com/scl/fi/lo60weugmmmzkg1w5aykd/Dirt_Line_2.jpg?rlkey=96nrdt0deouoxa85sv2bogdqx&raw=1

    It has some nice behavior around the diode capacitance feed-thru. At
    sample time T2, nodes D and L both slam to ground. Before then, they
    track.

    This slow sampler drives C1 hard, unlike a faster feedback sampler
    that can only poke a bit of incremental charge into the cap each shot.


    That one's nice too. How hard it drives the sampling cap depends on the
    video resistance of the diodes. BAT1502s seem to work okay.

    At the moment I'm trying to do it with just BFP840s and diodes, to make
    sure I understand all the details.

    I'm someplace between you and Joerg on the ideal functionality-vs-BOM
    cost continuum--some years ago ChesterW and I did a 100-ps class TDR
    sampler for a fuel gauge that sells for $25. My bit was well under $2, including the fast pulse generator, 1-diode first stage sampler, fast
    buffer amp, second stage sampler, and slow output buffer.

    It worked great, for a gas gauge--in fact CW and Co. Muntzed it a bit
    and got a few cents of cost out, in exchange for turning it into a
    200-ps sampler, which was better than good enough.

    The present one needs good waveform fidelity because we're basically
    measuring the shape of the drooly return pulse and converting that to
    the water and conductivity measurement.

    The spherical cows think that a FIN1002 driving a BFP840 can easily make
    a < 20 ps edge. I may disagree with that, but at least it's encouraging. ;)

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    http://hobbs-eo.com

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From john larkin @21:1/5 to pcdhSpamMeSenseless@electrooptical. on Tue Jul 9 17:19:21 2024
    On Tue, 9 Jul 2024 16:45:07 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-06 15:03, john larkin wrote:
    On Sat, 6 Jul 2024 11:56:08 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-06 00:46, john larkin wrote:
    On Fri, 5 Jul 2024 21:35:29 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 20:48, john larkin wrote:
    On Fri, 5 Jul 2024 20:41:01 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 16:30, Phil Hobbs wrote:
    On 2024-07-05 16:05, john larkin wrote:> On Fri, 5 Jul 2024 15:52:20 >>>>>>>> -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    On 2024-07-05 12:54, john larkin wrote:
    On Thu, 04 Jul 2024 20:24:38 -0700, john larkin
    <jlarkin_highland_tech> wrote:

    On Thu, 4 Jul 2024 23:12:43 -0000 (UTC), Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    john larkin <jlarkin_highland_tech> wrote:
    On Thu, 4 Jul 2024 21:10:20 -0000 (UTC), Phil Hobbs >>>>>>>>>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:

    boB <boB@K7IQ.com> wrote:
    On Wed, 3 Jul 2024 21:49:10 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>>>>>>>>>>>>
    So I have this new gig making a very low cost TDR >>>>>>>>>>>>>>>>> system for soil moisture and conductivity, for use >>>>>>>>>>>>>>>>> in agriculture.  All very interesting and topical, >>>>>>>>>>>>>>>>> what with droughts and low aquifers and all. >>>>>>>>>>>>>>>>>
    As one does, I'm planning to use a fast ramp and two >>>>>>>>>>>>>>>>> comparators to generate the TX pulse and the >>>>>>>>>>>>>>>>> sampling gate.  One will have a fixed comparison >>>>>>>>>>>>>>>>> voltage, and the other one's will be set by a DAC, or >>>>>>>>>>>>>>>>> possibly by a slower ramp, depending on the BOM vs >>>>>>>>>>>>>>>>> performance tradeoff we wind up with.

    Sooo, naturally I pulled out my fave LVDS line >>>>>>>>>>>>>>>>> receiver, the FIN1002. It has nice 250-300 ps edges >>>>>>>>>>>>>>>>> and pretty low jitter, comes in SOT23, and and costs >>>>>>>>>>>>>>>>> 30 cents on LCSC.

    It even has SPICE models, but of course they're >>>>>>>>>>>>>>>>> nasty encrypted HSPICE things.

    TI's seem to be the same.

    Anybody got a nice LVDS receiver with a real SPICE >>>>>>>>>>>>>>>>> model that mere mortals are allowed to use?

    Thanks

    Phil Hobbs


    Does the spec sheet say anything about how it works so >>>>>>>>>>>>>>>> that a model could be made ?  Block diagram or >>>>>>>>>>>>>>>> something ?

    boB



    Too much like work?it?ll be much easier to build the >>>>>>>>>>>>>>> circuit and measure it. I can simulate most of the fast >>>>>>>>>>>>>>> stuff, because it?s all discrete.

    Thanks

    Phil Hobbs

    What sorts of risetimes and swings do you want?

    I haven't used the FIN1002, but we've used several similar LVDS >>>>>>>>>>>>>> receivers and all seem to work like decent RRIO comparators. You >>>>>>>>>>>>>> can put an RC ramp into one input and a
    DAC into the other and make a ps-resolution programmable >>>>>>>>>>>>>> delay.

    I've given up on fast linear ramps... too much work. An RC >>>>>>>>>>>>>> with a bit of polynomial DAC correction works great. >>>>>>>>>>>>>
    I’m planning to use some fast gain to sharpen up the edge, >>>>>>>>>>>>> probably a BFP740. It won’t have much chance to oscillate, so >>>>>>>>>>>>> it should be fine.

    You have to be a bit careful, because the prop delay depends >>>>>>>>>>>>> some on the CM voltage. It’s a bit like the offset voltage of >>>>>>>>>>>>> a RRIO op amp—squirrely things happen within a couple of >>>>>>>>>>>>> V_BEs of one rail.


    The LVDS receivers that we use do behave much better when the >>>>>>>>>>>> inputs are a volt or so below Vcc.

    But we just poke in a bunch of DAC voltages, measure the delay >>>>>>>>>>>> times, and calculate a polynomial. Ramp curvature and cmrr >>>>>>>>>>>> don't matter much anymore.

    Take a look at the SY88022 laser driver chip. It's stunning. >>>>>>>>>>>> Diff in, brutal but clean 25 ps outputs, smooth amplitude >>>>>>>>>>>> control down to zero, about $7.


    Nice part, thanks.

    Or slam one or two SAV541 types with the FIN output. They go >>>>>>>>>>> from off to about 2 ohms in a few tenths of a volt of gate drive. >>>>>>>>>>> The newer packages should have less wirebond inductance than the >>>>>>>>>>> originals.

    Package inductance probably dominates actual fast switching >>>>>>>>>>> behavior. Spice models usually ignore that. I suppose that >>>>>>>>>>> inductances could be inferred from s-params, but I couldn't do >>>>>>>>>>> that.

    For a SOT23, if you figure 0.7 nH in series and 1/4 pF to ground, >>>>>>>>>> you won't go far wrong.  Leo Bodnar has gone upmarket, so I may >>>>>>>>>> need to start using them--I've used his stuff in POC protos, but >>>>>>>>>> not any more.

    With SiGe BJTs running in normal bias, the real key is keeping the >>>>>>>>>> pad capacitance down.  A good bead right at the base tames them >>>>>>>>>> very nicely at lowish drain current, but if you let the base pad >>>>>>>>>> capacitance get as large as 0.5 pF,  above about 5 mA you're toast >>>>>>>>>> regardless of how good the bead is.

    The nice thing about them for this job is that they cost $0.16 >>>>>>>>>> rather than $2.  (Part of the royalty conversation is BOM
    savings.)

    The business end of the TDR looks like a giant meat fork, with a >>>>>>>>>> slide hammer for getting it in and out of the ground.
    (Fortunately the circuit board is at the other end of a cable from >>>>>>>>>> the slide hammer!) It's got some sort of balun inside, which I hope >>>>>>>>>> is a bazooka-style one with ferrite, rather than something more >>>>>>>>>> complicated.

    You really only care about suppressing the current on the outside >>>>>>>>>> of the shield.  The classic bazooka is a 1/4-wave coaxial sleeve, >>>>>>>>>> shorted to the outer conductor at the generator end, so that it >>>>>>>>>> looks like an open circuit to any outside shield current.

    It's pretty narrowband, of course, but a ferrite sleeve can do the >>>>>>>>>> job pretty well--all you really care about is that the outside >>>>>>>>>> shield current sees a high impedance, and the phase angle doesn't >>>>>>>>>> matter much.

    Having control over both the TX and RX means that you can ignore a >>>>>>>>>> lot of stuff that real sampler designs have to worry about. >>>>>>>>>>
    Cheers

    Phil Hobbs

    You can make a TDR that's coyote ugly, as long as it's fast, and >>>>>>>>> deconvolve a software FIR filter to beautify it. And speed it up a >>>>>>>>> bit maybe.


    https://www.dropbox.com/scl/fi/ldhy31dlh5afpzc88pamr/TDR_Decon_demo.jpg?rlkey=69wt52psd8s7c7so1wl3tvl0c&raw=1



    Are you, of all people, suggesting that we use that nasty evil ill-posed
    ill-conditioned _deconvolution_ thing you used to beat me up about? >>>>>>>>
    Nah, must be my ear wax. ;)

    If I can come up with a good online cal scheme, that's potentially a >>>>>>> helpful method. I really don't want to have to be Agoston Agoston. ;) >>>>>>>
    But you can make things like blowby, nonlinear capacitance, poor >>>>>>> sampling efficiency due to the diode R * sampling C time constant being >>>>>>> too slow, and so on, go away.

    This is done by (1) doing the second-stage sampling when nothing is >>>>>>> going on at the input. That fixes the blowby problem.

    Then (2), you feed back the second-stage sampled value to the sampling >>>>>>> cap, which is the normal sampling loop approach.

    Finally (3), you take 20 samples per delay value, so that the sampling >>>>>>> loop converges, resulting in zero volts across the sampling bridge at >>>>>>> the sampling instant. That gets rid of the nonlinear capacitance problem.

    Cheers

    Phil Hobbs

    What sort of rise time are you after?

    There are goofy possibilities.


    For TDRing dirt, I find it hard to believe we'll need better than 100 ps. >>>

    Something sort of like this might work.

    https://www.dropbox.com/scl/fi/yq7an2udjygxrd2bf0rdl/DirtLine_TDR.jpg?rlkey=jyu9gb74y6dciml307jpad8bw&raw=1


    Fun. I like it in general--it should have a nice clean Tx edge, for one >>> thing, and the ON resistance of the pHEMT is a lot better than a
    Schottky diode for the same capacitance. That might eliminate the need
    to use multiple sampling pulses to get the sampling loop to converge.
    (Which would be good, because you can't easily stack samples with a
    track/hold architecture.)

    ESD protection is one possible issue, and the hold capacitor loads down
    the line throughout the pulse and return, so it has to be very small.

    So far I'm envisioning a two-diode series sampler, with a fast sampling
    pulse made by taking a fast edge from a BFP840 and differentiating it
    with a shorted Tx line, maybe with a bit of capacitive help.

    As long as the final voltages across the diodes don't change from sample >>> to sample, the nonlinear capacitance funnies pretty well go away. There >>> might be some schmutz due to reflections off the bridge in the OFF state >>> and other discontinuities rattling round. It'll be hard to get rid of
    the reflection from where the tines hit the soil surface, for instance.

    As I say, fun.

    Cheers

    Phil Hobbs

    How about this?

    https://www.dropbox.com/scl/fi/lo60weugmmmzkg1w5aykd/Dirt_Line_2.jpg?rlkey=96nrdt0deouoxa85sv2bogdqx&raw=1

    It has some nice behavior around the diode capacitance feed-thru. At
    sample time T2, nodes D and L both slam to ground. Before then, they
    track.

    This slow sampler drives C1 hard, unlike a faster feedback sampler
    that can only poke a bit of incremental charge into the cap each shot.


    That one's nice too. How hard it drives the sampling cap depends on the >video resistance of the diodes. BAT1502s seem to work okay.

    A BAT1502 should be well below 10 ohms, which into a few pF will be
    plenty fast.



    At the moment I'm trying to do it with just BFP840s and diodes, to make
    sure I understand all the details.

    I'm someplace between you and Joerg on the ideal functionality-vs-BOM
    cost continuum--some years ago ChesterW and I did a 100-ps class TDR
    sampler for a fuel gauge that sells for $25. My bit was well under $2, >including the fast pulse generator, 1-diode first stage sampler, fast
    buffer amp, second stage sampler, and slow output buffer.

    It worked great, for a gas gauge--in fact CW and Co. Muntzed it a bit
    and got a few cents of cost out, in exchange for turning it into a
    200-ps sampler, which was better than good enough.

    The present one needs good waveform fidelity because we're basically >measuring the shape of the drooly return pulse and converting that to
    the water and conductivity measurement.

    The spherical cows think that a FIN1002 driving a BFP840 can easily make
    a < 20 ps edge. I may disagree with that, but at least it's encouraging. ;)

    Cheers

    Phil Hobbs

    Sounds ambitious, but if you try it I'd love to know what happens.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)