• Time-Domain Reflectometry: debugging 2.7 GHz oscillations and Murata fe

    From Phil Hobbs@21:1/5 to All on Thu Mar 31 12:09:19 2022
    So I've been wringing out a new APD front end. It's a 500-MHz TIA with
    a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a strong squeg
    at about 1 MHz. Changing the drain current changes the duty cycle of
    the squegging, and reducing it below about 2 mA makes it stable.

    The oscillation frequency doesn't change much (5% or so) with drain
    current. All of which suggests that the oscillation is due to some reasonably-sharp resonance someplace--the squegging gets worse at higher
    gain, but the oscillation frequency doesn't move around. The waveform
    is more or less sinusoidal-looking, but that's not surprising since the vertical bandwidth of the scope (a TDS 694C) is about 3.3 GHz. The
    oscillation doesn't depend on whether the TIA is connected or not.

    Lower-speed front ends based on the same sort of transistor are famously stable--one of them runs the bootstrap across a two-inch-long FFC cable
    going to a MPPC on a cold plate. They also work great as the bottom
    device in a cascode with a 45-GHz SiGe NPN. (The NPN needs a
    base-stopper bead, but the pHEMT doesn't.)

    The difference with this one may be that there's a bootstrapped pour
    under the summing junction components, driven by the pHEMT source.

    Sooo, I took a bare board, bodged in a U.FL micro coax connector from
    the bootstrapped pour to ground, and hung it on the front of one of my
    trusty Tek SD-24 TDRs, like so: <https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

    (U.FLs are super useful for this sort of thing--far better than coax
    pigtails. Not bad for 20 cents.)

    Here's the result: <https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png>
    (green curve: U.FL unplugged; white curve: board attached).

    There's a capacitive dip (3.0 pF as measured on a Boonton 72BD) followed
    by a delayed and not-too step open-circuit reflection and some ringing. Interestingly the reciprocal of the round-trip delay is right around 2.7
    GHz, which would make sense with an open-circuit transmission line
    resonator.

    I also did some measurements of the Murata beads we use as base/gate
    stoppers for microwave transistors. Our faves are the Murata BLM1xBA
    series.

    The following scope photo shows a TDR of a short piece of 0.080"
    hardline with various low-Z Murata ferrite beads. From top to bottom at
    the beginning of the falling-edge transient:
    BLM18BA100SN1D (10 ohms @ 100 MHz, light orange);
    BLM15BA050SN1D (5 ohms @ 100 MHz, green);
    BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
    BLM15BB050SN1D (10 ohms @ 100 MHz, yellowish).

    <https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png>

    The BA-series beads show a lot better high-frequency impedance than the BB-series ones, despite their datasheet curves being very similar. The BLM15BA050 shows a bit of undershoot near 1.5 ns, but the others are all basically monotonic at late times.

    Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical at
    this resolution--the traces lie right on top of each other.

    Fun stuff, and it'll be more fun once I get the resonance problem
    knocked. Putting the pHEMT source connection near the middle of the
    pour instead of at one end will help, I expect--this part ought to be
    less likely to oscillate at 5 GHz.

    Comments welcome.

    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 Mar 31 09:58:32 2022
    On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I've been wringing out a new APD front end. It's a 500-MHz TIA with
    a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a strong squeg
    at about 1 MHz. Changing the drain current changes the duty cycle of
    the squegging, and reducing it below about 2 mA makes it stable.

    The oscillation frequency doesn't change much (5% or so) with drain
    current. All of which suggests that the oscillation is due to some >reasonably-sharp resonance someplace--the squegging gets worse at higher >gain, but the oscillation frequency doesn't move around. The waveform
    is more or less sinusoidal-looking, but that's not surprising since the >vertical bandwidth of the scope (a TDS 694C) is about 3.3 GHz. The >oscillation doesn't depend on whether the TIA is connected or not.

    Lower-speed front ends based on the same sort of transistor are famously >stable--one of them runs the bootstrap across a two-inch-long FFC cable
    going to a MPPC on a cold plate. They also work great as the bottom
    device in a cascode with a 45-GHz SiGe NPN. (The NPN needs a
    base-stopper bead, but the pHEMT doesn't.)

    The difference with this one may be that there's a bootstrapped pour
    under the summing junction components, driven by the pHEMT source.

    Sooo, I took a bare board, bodged in a U.FL micro coax connector from
    the bootstrapped pour to ground, and hung it on the front of one of my
    trusty Tek SD-24 TDRs, like so: ><https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

    (U.FLs are super useful for this sort of thing--far better than coax >pigtails. Not bad for 20 cents.)

    Here's the result: ><https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png>
    (green curve: U.FL unplugged; white curve: board attached).

    There's a capacitive dip (3.0 pF as measured on a Boonton 72BD) followed
    by a delayed and not-too step open-circuit reflection and some ringing. >Interestingly the reciprocal of the round-trip delay is right around 2.7
    GHz, which would make sense with an open-circuit transmission line
    resonator.

    I also did some measurements of the Murata beads we use as base/gate
    stoppers for microwave transistors. Our faves are the Murata BLM1xBA
    series.

    The following scope photo shows a TDR of a short piece of 0.080"
    hardline with various low-Z Murata ferrite beads. From top to bottom at
    the beginning of the falling-edge transient:
    BLM18BA100SN1D (10 ohms @ 100 MHz, light orange);
    BLM15BA050SN1D (5 ohms @ 100 MHz, green);
    BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
    BLM15BB050SN1D (10 ohms @ 100 MHz, yellowish).

    <https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png>

    The BA-series beads show a lot better high-frequency impedance than the >BB-series ones, despite their datasheet curves being very similar. The >BLM15BA050 shows a bit of undershoot near 1.5 ns, but the others are all >basically monotonic at late times.

    Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical at
    this resolution--the traces lie right on top of each other.

    Fun stuff, and it'll be more fun once I get the resonance problem
    knocked. Putting the pHEMT source connection near the middle of the
    pour instead of at one end will help, I expect--this part ought to be
    less likely to oscillate at 5 GHz.

    Comments welcome.

    Cheers

    Phil Hobbs

    A bootstrapped pour seems like a great idea. I did that on my
    triggered Colpitts oscillator to reduce the effective/terrible FR4
    capacitance. But it made things unstable for some reason, so I
    reverted to cutting some chunks out of the ground and power planes and temperature compensating out the positive TC of the FR4.

    A couple of those bead TDRs look like capacitors to me.

    I like gate resistors to tame phemts, but we mostly work with big
    signals so a few more nV of noise is no big deal to us. I'm using 499
    ohms in one case! I guess a hi-Z bead makes as much hf Johnson noise
    as a resistor.

    Gotta study your post in more detail when I have more time.

    TDR rocks.
    --

    If a man will begin with certainties, he shall end with doubts,
    but if he will be content to begin with doubts he shall end in certainties. Francis Bacon

    --- SoupGate-Win32 v1.05
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  • From Gerhard Hoffmann@21:1/5 to All on Thu Mar 31 19:33:45 2022
    Am 31.03.22 um 18:58 schrieb John Larkin:
    On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I've been wringing out a new APD front end. It's a 500-MHz TIA with
    a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a strong squeg
    at about 1 MHz. Changing the drain current changes the duty cycle of
    the squegging, and reducing it below about 2 mA makes it stable.

    The oscillation frequency doesn't change much (5% or so) with drain
    current. All of which suggests that the oscillation is due to some
    reasonably-sharp resonance someplace--the squegging gets worse at higher
    gain, but the oscillation frequency doesn't move around. The waveform
    is more or less sinusoidal-looking, but that's not surprising since the
    vertical bandwidth of the scope (a TDS 694C) is about 3.3 GHz. The
    oscillation doesn't depend on whether the TIA is connected or not.

    Lower-speed front ends based on the same sort of transistor are famously
    stable--one of them runs the bootstrap across a two-inch-long FFC cable
    going to a MPPC on a cold plate. They also work great as the bottom
    device in a cascode with a 45-GHz SiGe NPN. (The NPN needs a
    base-stopper bead, but the pHEMT doesn't.)

    The difference with this one may be that there's a bootstrapped pour
    under the summing junction components, driven by the pHEMT source.

    Sooo, I took a bare board, bodged in a U.FL micro coax connector from
    the bootstrapped pour to ground, and hung it on the front of one of my
    trusty Tek SD-24 TDRs, like so:
    <https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

    (U.FLs are super useful for this sort of thing--far better than coax
    pigtails. Not bad for 20 cents.)

    Here's the result:
    <https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png>
    (green curve: U.FL unplugged; white curve: board attached).

    There's a capacitive dip (3.0 pF as measured on a Boonton 72BD) followed
    by a delayed and not-too step open-circuit reflection and some ringing.
    Interestingly the reciprocal of the round-trip delay is right around 2.7
    GHz, which would make sense with an open-circuit transmission line
    resonator.

    I also did some measurements of the Murata beads we use as base/gate
    stoppers for microwave transistors. Our faves are the Murata BLM1xBA
    series.

    The following scope photo shows a TDR of a short piece of 0.080"
    hardline with various low-Z Murata ferrite beads. From top to bottom at
    the beginning of the falling-edge transient:
    BLM18BA100SN1D (10 ohms @ 100 MHz, light orange);
    BLM15BA050SN1D (5 ohms @ 100 MHz, green);
    BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
    BLM15BB050SN1D (10 ohms @ 100 MHz, yellowish).

    <https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png>

    The BA-series beads show a lot better high-frequency impedance than the
    BB-series ones, despite their datasheet curves being very similar. The
    BLM15BA050 shows a bit of undershoot near 1.5 ns, but the others are all
    basically monotonic at late times.

    Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical at
    this resolution--the traces lie right on top of each other.

    Fun stuff, and it'll be more fun once I get the resonance problem
    knocked. Putting the pHEMT source connection near the middle of the
    pour instead of at one end will help, I expect--this part ought to be
    less likely to oscillate at 5 GHz.

    Comments welcome.

    Cheers

    Phil Hobbs

    A bootstrapped pour seems like a great idea. I did that on my
    triggered Colpitts oscillator to reduce the effective/terrible FR4 capacitance. But it made things unstable for some reason, so I
    reverted to cutting some chunks out of the ground and power planes and temperature compensating out the positive TC of the FR4.

    A couple of those bead TDRs look like capacitors to me.

    I like gate resistors to tame phemts, but we mostly work with big
    signals so a few more nV of noise is no big deal to us. I'm using 499
    ohms in one case! I guess a hi-Z bead makes as much hf Johnson noise
    as a resistor.



    Yes, at frequencies where it works like a resistor, it creates
    noise like a resistor.

    V1 is only there as a compiler pleaser, needed for syntax.

    <
    https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public/
    >

    chears, Gerhard

    --- SoupGate-Win32 v1.05
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  • From Phil Hobbs@21:1/5 to Gerhard Hoffmann on Thu Mar 31 14:50:55 2022
    Gerhard Hoffmann wrote:
    Am 31.03.22 um 18:58 schrieb John Larkin:
    On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs
    <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I've been wringing out a new APD front end.  It's a 500-MHz TIA with >>> a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a strong squeg >>> at about 1 MHz.  Changing the drain current changes the duty cycle of
    the squegging, and reducing it below about 2 mA makes it stable.

    The oscillation frequency doesn't change much (5% or so) with drain
    current.  All of which suggests that the oscillation is due to some
    reasonably-sharp resonance someplace--the squegging gets worse at higher >>> gain, but the oscillation frequency doesn't move around.  The waveform
    is more or less sinusoidal-looking, but that's not surprising since the
    vertical bandwidth of the scope (a TDS 694C) is about 3.3 GHz.  The
    oscillation doesn't depend on whether the TIA is connected or not.

    Lower-speed front ends based on the same sort of transistor are famously >>> stable--one of them runs the bootstrap across a two-inch-long FFC cable
    going to a MPPC on a cold plate.  They also work great as the bottom
    device in a cascode with a 45-GHz SiGe NPN.  (The NPN needs a
    base-stopper bead, but the pHEMT doesn't.)

    The difference with this one may be that there's a bootstrapped pour
    under the summing junction components, driven by the pHEMT source.

    Sooo, I took a bare board, bodged in a U.FL micro coax connector from
    the bootstrapped pour to ground, and hung it on the front of one of my
    trusty Tek SD-24 TDRs, like so:
    <https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

    (U.FLs are super useful for this sort of thing--far better than coax
    pigtails.  Not bad for 20 cents.)

    Here's the result:
    <https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png>
    (green curve: U.FL unplugged; white curve: board attached).

    There's a capacitive dip (3.0 pF as measured on a Boonton 72BD) followed >>> by a delayed and not-too step open-circuit reflection and some ringing.
    Interestingly the reciprocal of the round-trip delay is right around 2.7 >>> GHz, which would make sense with an open-circuit transmission line
    resonator.

    I also did some measurements of the Murata beads we use as base/gate
    stoppers for microwave transistors.  Our faves are the Murata BLM1xBA
    series.

    The following scope photo shows a TDR of a short piece of 0.080"
    hardline with various low-Z Murata ferrite beads.  From top to bottom at >>> the beginning of the falling-edge transient:
    BLM18BA100SN1D (10 ohms @ 100 MHz, light orange);
    BLM15BA050SN1D (5 ohms @ 100 MHz, green);
    BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
    BLM15BB050SN1D (10 ohms @ 100 MHz, yellowish).

    <https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png>


    The BA-series beads show a lot better high-frequency impedance than the
    BB-series ones, despite their datasheet curves being very similar.  The >>> BLM15BA050 shows a bit of undershoot near 1.5 ns, but the others are all >>> basically monotonic at late times.

    Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical at
    this resolution--the traces lie right on top of each other.

    Fun stuff, and it'll be more fun once I get the resonance problem
    knocked.  Putting the pHEMT source connection near the middle of the
    pour instead of at one end will help, I expect--this part ought to be
    less likely to oscillate at 5 GHz.

    Comments welcome.

    Cheers

    Phil Hobbs

    A bootstrapped pour seems like a great idea. I did that on my
    triggered Colpitts oscillator to reduce the effective/terrible FR4
    capacitance. But it made things unstable for some reason, so I
    reverted to cutting some chunks out of the ground and power planes and
    temperature compensating out the positive TC of the FR4.

    A couple of those bead TDRs look like capacitors to me.

    I like gate resistors to tame phemts, but we mostly work with big
    signals so a few more nV of noise is no big deal to us. I'm using 499
    ohms in one case! I guess a hi-Z bead makes as much hf Johnson noise
    as a resistor.



    Yes, at frequencies where it works like a resistor, it creates
    noise like a resistor.

    V1 is only there as a compiler pleaser, needed for syntax.

    < https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public/
         >

    Comes up blank for me. But it's true--there's a very general theorem of classical thermodynamics that states that *in thermal equilibrium* any
    process that can dissipate power will also produce fluctuations with
    average power kT/2 per classical degree of freedom.

    You can derive the sqrt(4kTR) formula by considering an isolated RC
    circuit, setting the mean energy on the capacitor as kT/2, and computing
    what the resistor's noise power has to be in order to keep the mean
    energy constant.

    As soon as you apply power, of course, all the nice theorems go
    away--for instance, a diode-connected transistor looks like a resistor
    whose value is 25 mV/I_C, and whose temperature is T_J/2 (150 K at room temperature).

    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 Gerhard Hoffmann@21:1/5 to All on Thu Mar 31 21:11:30 2022
    Am 31.03.22 um 20:50 schrieb Phil Hobbs:
    Gerhard Hoffmann wrote:


    Yes, at frequencies where it works like a resistor, it creates
    noise like a resistor.

    V1 is only there as a compiler pleaser, needed for syntax.

    <
    https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public/
          >

    Comes up blank for me.

    Works for me. 2k5 * 1k5 screen dump.
    called up 7 times upto now.
    But Flickr is sloooooow!

    Gerhard

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to All on Thu Mar 31 12:59:11 2022
    On Thu, 31 Mar 2022 21:11:30 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
    wrote:

    Am 31.03.22 um 20:50 schrieb Phil Hobbs:
    Gerhard Hoffmann wrote:


    Yes, at frequencies where it works like a resistor, it creates
    noise like a resistor.

    V1 is only there as a compiler pleaser, needed for syntax.

    <
    https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public/
    >

    Comes up blank for me.

    Works for me. 2k5 * 1k5 screen dump.
    called up 7 times upto now.
    But Flickr is sloooooow!

    Gerhard


    Fine, fast here with Firefox.

    --

    If a man will begin with certainties, he shall end with doubts,
    but if he will be content to begin with doubts he shall end in certainties. Francis Bacon

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Doe@21:1/5 to Phil Hobbs on Thu Mar 31 19:24:06 2022
    Phil Hobbs wrote:

    Gerhard Hoffmann wrote:

    https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public

    Comes up blank for me.

    Here... Fails in Firefox. Works in Microsoft Edge.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From ehsjr@21:1/5 to Gerhard Hoffmann on Thu Mar 31 18:35:51 2022
    On 3/31/2022 3:11 PM, Gerhard Hoffmann wrote:
    Am 31.03.22 um 20:50 schrieb Phil Hobbs:
    Gerhard Hoffmann wrote:


    Yes, at frequencies where it works like a resistor, it creates
    noise like a resistor.

    V1 is only there as a compiler pleaser, needed for syntax.

    <
    https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public/
          >

    Comes up blank for me.

    Works for me. 2k5 * 1k5 screen dump.
    called up 7 times upto now.
    But Flickr is sloooooow!

    Gerhard


    Works for me too.
    Ed

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Phil Hobbs on Thu Mar 31 20:09:39 2022
    Phil Hobbs wrote:
    So I've been wringing out a new APD front end. It's a 500-MHz TIA
    with a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a
    strong squeg at about 1 MHz. Changing the drain current changes the
    duty cycle of the squegging, and reducing it below about 2 mA makes
    it stable.

    The oscillation frequency doesn't change much (5% or so) with drain
    current. All of which suggests that the oscillation is due to some reasonably-sharp resonance someplace--the squegging gets worse at
    higher gain, but the oscillation frequency doesn't move around. The
    waveform is more or less sinusoidal-looking, but that's not
    surprising since the vertical bandwidth of the scope (a TDS 694C) is
    about 3.3 GHz. The oscillation doesn't depend on whether the TIA is connected or not.

    Lower-speed front ends based on the same sort of transistor are
    famously stable--one of them runs the bootstrap across a
    two-inch-long FFC cable going to a MPPC on a cold plate. They also
    work great as the bottom device in a cascode with a 45-GHz SiGe NPN.
    (The NPN needs a base-stopper bead, but the pHEMT doesn't.)

    The difference with this one may be that there's a bootstrapped pour
    under the summing junction components, driven by the pHEMT source.

    Sooo, I took a bare board, bodged in a U.FL micro coax connector from
    the bootstrapped pour to ground, and hung it on the front of one of
    my trusty Tek SD-24 TDRs, like so: <https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

    (U.FLs are super useful for this sort of thing--far better than coax
    pigtails. Not bad for 20 cents.)

    Here's the result: <https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png>
    (green curve: U.FL unplugged; white curve: board attached).

    There's a capacitive dip (3.0 pF as measured on a Boonton 72BD)
    followed by a delayed and not-too-
    steep
    open-circuit reflection and some ringing. Interestingly the
    reciprocal of the round-trip delay is right around 2.7 GHz, which
    would make sense with an open-circuit transmission line resonator.

    I also did some measurements of the Murata beads we use as base/gate
    stoppers for microwave transistors. Our faves are the Murata
    BLM1xBA series.

    The following scope photo shows a TDR of a short piece of 0.080"
    hardline with various low-Z Murata ferrite beads. From top to bottom
    at the beginning of the falling-edge transient: BLM18BA100SN1D (10
    ohms @ 100 MHz, light orange); BLM15BA050SN1D (5 ohms @ 100 MHz,
    green); BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
    BLM15BB050SN1D (
    5
    ohms @ 100 MHz, yellowish).

    <https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png>



    The BA-series beads show a lot better high-frequency impedance than
    the BB-series ones, despite their datasheet curves being very
    similar. The BLM15BA050 shows a bit of undershoot near 1.5 ns, but
    the others are all basically monotonic at late times.

    Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical
    at this resolution--the traces lie right on top of each other.

    Fun stuff, and it'll be more fun once I get the resonance problem
    knocked. Putting the pHEMT source connection near the middle of the
    pour instead of at one end will help, I expect--this part ought to
    be less likely to oscillate at 5 GHz.

    Comments welcome.

    (Fixed a couple of typos that might impede understanding)

    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 Clifford Heath@21:1/5 to Phil Hobbs on Fri Apr 1 10:32:24 2022
    On 1/4/22 3:09 am, Phil Hobbs wrote:
    Comments welcome.

    No technical comment, just this:

    Fascinating post, many thanks for sharing.

    CH

    --- SoupGate-Win32 v1.05
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  • From Phil Hobbs@21:1/5 to John Larkin on Fri Apr 1 08:42:50 2022
    John Larkin wrote:
    On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    So I've been wringing out a new APD front end. It's a 500-MHz TIA with
    a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a strong squeg
    at about 1 MHz. Changing the drain current changes the duty cycle of
    the squegging, and reducing it below about 2 mA makes it stable.

    The oscillation frequency doesn't change much (5% or so) with drain
    current. All of which suggests that the oscillation is due to some
    reasonably-sharp resonance someplace--the squegging gets worse at higher
    gain, but the oscillation frequency doesn't move around. The waveform
    is more or less sinusoidal-looking, but that's not surprising since the
    vertical bandwidth of the scope (a TDS 694C) is about 3.3 GHz. The
    oscillation doesn't depend on whether the TIA is connected or not.

    Lower-speed front ends based on the same sort of transistor are famously
    stable--one of them runs the bootstrap across a two-inch-long FFC cable
    going to a MPPC on a cold plate. They also work great as the bottom
    device in a cascode with a 45-GHz SiGe NPN. (The NPN needs a
    base-stopper bead, but the pHEMT doesn't.)

    The difference with this one may be that there's a bootstrapped pour
    under the summing junction components, driven by the pHEMT source.

    Sooo, I took a bare board, bodged in a U.FL micro coax connector from
    the bootstrapped pour to ground, and hung it on the front of one of my
    trusty Tek SD-24 TDRs, like so:
    <https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

    (U.FLs are super useful for this sort of thing--far better than coax
    pigtails. Not bad for 20 cents.)

    Here's the result:
    <https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png>
    (green curve: U.FL unplugged; white curve: board attached).

    There's a capacitive dip (3.0 pF as measured on a Boonton 72BD) followed
    by a delayed and not-too step open-circuit reflection and some ringing.
    Interestingly the reciprocal of the round-trip delay is right around 2.7
    GHz, which would make sense with an open-circuit transmission line
    resonator.

    I also did some measurements of the Murata beads we use as base/gate
    stoppers for microwave transistors. Our faves are the Murata BLM1xBA
    series.

    The following scope photo shows a TDR of a short piece of 0.080"
    hardline with various low-Z Murata ferrite beads. From top to bottom at
    the beginning of the falling-edge transient:
    BLM18BA100SN1D (10 ohms @ 100 MHz, light orange);
    BLM15BA050SN1D (5 ohms @ 100 MHz, green);
    BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
    BLM15BB050SN1D (10 ohms @ 100 MHz, yellowish).

    <https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png>

    The BA-series beads show a lot better high-frequency impedance than the
    BB-series ones, despite their datasheet curves being very similar. The
    BLM15BA050 shows a bit of undershoot near 1.5 ns, but the others are all
    basically monotonic at late times.

    Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical at
    this resolution--the traces lie right on top of each other.

    Fun stuff, and it'll be more fun once I get the resonance problem
    knocked. Putting the pHEMT source connection near the middle of the
    pour instead of at one end will help, I expect--this part ought to be
    less likely to oscillate at 5 GHz.

    Comments welcome.


    A bootstrapped pour seems like a great idea. I did that on my
    triggered Colpitts oscillator to reduce the effective/terrible FR4 capacitance. But it made things unstable for some reason, so I
    reverted to cutting some chunks out of the ground and power planes and temperature compensating out the positive TC of the FR4.

    A couple of those bead TDRs look like capacitors to me.

    The higher-Z BB-series parts are more resistive than the BA-series
    ones--poking around on these pages is pretty illuminating.

    <https://ds.murata.co.jp/simsurfing/gateway.html?partnumbers=%5B%22BLM18AB050SN1D%22%5D&rgear=suaykx&rgearinfo=com>

    (You can replace the part number with the one you actually want to
    see--getting the same plot by dorking with the web page is much slower.)

    A five-ohm bead that peaks at nearly 100 ohms resisitive out at 4 GHz or
    so is a super useful part.



    I like gate resistors to tame phemts, but we mostly work with big
    signals so a few more nV of noise is no big deal to us. I'm using 499
    ohms in one case! I guess a hi-Z bead makes as much hf Johnson noise
    as a resistor.

    Gotta study your post in more detail when I have more time.

    TDR rocks.


    Yup, especially for time-domain stuff. Stuff like stability analysis is
    a bit subtler using TDR vs. a network analyzer. I should break out my
    HP 70820A (a 40 GHz scope/network analyzer/spectrum analyzer based on samplers).

    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

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  • From Cursitor Doom@21:1/5 to All on Sun Apr 3 17:59:28 2022
    On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    [...]

    Obviously you do a *lot* of testing and development, Phil. Which
    particular area of electronics do you find most stimulating to tackle?
    (if any nowadays given your extreme level of familiarity with the
    subject, that is).

    --- SoupGate-Win32 v1.05
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  • From Phil Hobbs@21:1/5 to Cursitor Doom on Sun Apr 3 21:15:45 2022
    Cursitor Doom wrote:
    On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    [...]

    Obviously you do a *lot* of testing and development, Phil. Which
    particular area of electronics do you find most stimulating to tackle?
    (if any nowadays given your extreme level of familiarity with the
    subject, that is).


    I mostly like building complete instruments. I've been doing a lot of
    front ends lately, which is getting boring, so we're trying to pour a
    bunch of concrete so that we can turn round a customized front end for
    folks made from nice tested building blocks.

    I thought this oscillation thing was fairly interesting, and was looking
    for some input from other folks. (Maybe even Joerg, if I can entice him
    away from doing fast laps around his bierkeller on the mountain bike. ;)

    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)