• Re: DDS filters

    From piglet@21:1/5 to john larkin on Tue Sep 17 22:57:57 2024
    john larkin <jl@650pot.com> wrote:
    I can use an Efinix FPGA and a bunch of cheap fast DACs to make some
    DDS clock sources, specifically four. The pain is the lowpass filter.

    Mini-Circuits and other folks make nice surface-mount lowpass filters,
    but they are most all in the GHz range. I want maybe 25 MHz. You'd
    think there would be a market for packaged MHz-range lowpsss filters.

    It's worth pushing the DAC rate as high as possible to simplify the
    lowpass filter. Stay far away from Nyquist.





    Wouldn’t your nu- hertz package create a filter out of a few 0603 parts far cheaper than a bought in filter?

    --
    piglet

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  • From john larkin@21:1/5 to All on Tue Sep 17 15:48:15 2024
    I can use an Efinix FPGA and a bunch of cheap fast DACs to make some
    DDS clock sources, specifically four. The pain is the lowpass filter.

    Mini-Circuits and other folks make nice surface-mount lowpass filters,
    but they are most all in the GHz range. I want maybe 25 MHz. You'd
    think there would be a market for packaged MHz-range lowpsss filters.

    It's worth pushing the DAC rate as high as possible to simplify the
    lowpass filter. Stay far away from Nyquist.

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  • From john larkin@21:1/5 to erichpwagner@hotmail.com on Tue Sep 17 16:38:42 2024
    On Tue, 17 Sep 2024 22:57:57 -0000 (UTC), piglet
    <erichpwagner@hotmail.com> wrote:

    john larkin <jl@650pot.com> wrote:
    I can use an Efinix FPGA and a bunch of cheap fast DACs to make some
    DDS clock sources, specifically four. The pain is the lowpass filter.

    Mini-Circuits and other folks make nice surface-mount lowpass filters,
    but they are most all in the GHz range. I want maybe 25 MHz. You'd
    think there would be a market for packaged MHz-range lowpsss filters.

    It's worth pushing the DAC rate as high as possible to simplify the
    lowpass filter. Stay far away from Nyquist.





    Wouldn’t your nu- hertz package create a filter out of a few 0603 parts far >cheaper than a bought in filter?

    Sure, but that will take a bunch of parts on the board, times four.
    The filters will be bigger than the DACs.

    I'm updating an old VME board design, a good seller full of obsolete
    parts. It used an Analog Devices DDS chip, and the filter has four
    parts, LCLC, which isn't too awful. But some little ceramic LTCC
    thingie would be great.

    Some people make ferrite-bead sorts of things, 3-terminal emi filters
    with caps inside. I'll look into those. Might get lucky.

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  • From Bill Sloman@21:1/5 to john larkin on Wed Sep 18 16:48:36 2024
    On 18/09/2024 8:48 am, john larkin wrote:
    I can use an Efinix FPGA and a bunch of cheap fast DACs to make some
    DDS clock sources, specifically four. The pain is the lowpass filter.

    Mini-Circuits and other folks make nice surface-mount lowpass filters,
    but they are most all in the GHz range. I want maybe 25 MHz. You'd
    think there would be a market for packaged MHz-range lowpsss filters.

    It's worth pushing the DAC rate as high as possible to simplify the
    lowpass filter. Stay far away from Nyquist.

    That kind of circuit cries out for finite impulse response low pass filter.

    You feed the digital signal through a shift register and hang sampling resistors on each tap, and sum the currents fed through the resistors.
    You do have to watch out for truncation error - Gibb's oscillations -
    and use a Hamming window when you calculate the value for each sampling resistor.

    The neat thing about it is that it is essentially frequency independent
    - the cut -off frequency scales with the clock rate.

    It's sort of bulky - my 32-stage example need two or three E-96
    precision resistors per tap to get the precision you need, but in
    surface mount that's tolerable.

    Shorter shift registers don't cut off as sharply but can still do much
    better than analog parts.

    --
    Bill Sloman, Sydney

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  • From john larkin@21:1/5 to All on Wed Sep 18 11:04:09 2024
    On Wed, 18 Sep 2024 16:48:36 +1000, Bill Sloman <bill.sloman@ieee.org>
    wrote:

    On 18/09/2024 8:48 am, john larkin wrote:
    I can use an Efinix FPGA and a bunch of cheap fast DACs to make some
    DDS clock sources, specifically four. The pain is the lowpass filter.

    Mini-Circuits and other folks make nice surface-mount lowpass filters,
    but they are most all in the GHz range. I want maybe 25 MHz. You'd
    think there would be a market for packaged MHz-range lowpsss filters.

    It's worth pushing the DAC rate as high as possible to simplify the
    lowpass filter. Stay far away from Nyquist.

    That kind of circuit cries out for finite impulse response low pass filter.

    You feed the digital signal through a shift register and hang sampling >resistors on each tap, and sum the currents fed through the resistors.
    You do have to watch out for truncation error - Gibb's oscillations -
    and use a Hamming window when you calculate the value for each sampling >resistor.

    The neat thing about it is that it is essentially frequency independent
    - the cut -off frequency scales with the clock rate.

    It's sort of bulky - my 32-stage example need two or three E-96
    precision resistors per tap to get the precision you need, but in
    surface mount that's tolerable.

    Shorter shift registers don't cut off as sharply but can still do much
    better than analog parts.

    It's interesting that there is a class of people who want to do
    totally impractical expensive things on circuit boards. People with no
    common sense. The name for such people is "fired."

    Also, a DDS lowpass filter can have ghasty passband response. What
    matters is stopband rejection. All the classic filter responses try to
    optimize passband flatness.

    The jitter of a DDS at low frequencies is domnated by the number of
    MSB bits that we pick from the phase accumulator. It's usually better
    to synthesize a clean octave and divide down as needed.

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  • From Bill Sloman@21:1/5 to john larkin on Thu Sep 19 13:49:27 2024
    On 19/09/2024 4:04 am, john larkin wrote:
    On Wed, 18 Sep 2024 16:48:36 +1000, Bill Sloman <bill.sloman@ieee.org>
    wrote:

    On 18/09/2024 8:48 am, john larkin wrote:
    I can use an Efinix FPGA and a bunch of cheap fast DACs to make some
    DDS clock sources, specifically four. The pain is the lowpass filter.

    Mini-Circuits and other folks make nice surface-mount lowpass filters,
    but they are most all in the GHz range. I want maybe 25 MHz. You'd
    think there would be a market for packaged MHz-range lowpsss filters.

    It's worth pushing the DAC rate as high as possible to simplify the
    lowpass filter. Stay far away from Nyquist.

    That kind of circuit cries out for finite impulse response low pass filter. >>
    You feed the digital signal through a shift register and hang sampling
    resistors on each tap, and sum the currents fed through the resistors.
    You do have to watch out for truncation error - Gibb's oscillations -
    and use a Hamming window when you calculate the value for each sampling
    resistor.

    The neat thing about it is that it is essentially frequency independent
    - the cut -off frequency scales with the clock rate.

    It's sort of bulky - my 32-stage example need two or three E-96
    precision resistors per tap to get the precision you need, but in
    surface mount that's tolerable.

    Shorter shift registers don't cut off as sharply but can still do much
    better than analog parts.

    It's interesting that there is a class of people who want to do
    totally impractical expensive things on circuit boards. People with no
    common sense. The name for such people is "fired."

    It's depressing that there is a class of people who suffer from "not
    invented here" and complain that anything that they didn't think of is impractical and expensive.


    Also, a DDS lowpass filter can have ghasty passband response.

    If cobbled together by the likes of John Larkin. The sort of people who
    can get ghastly jitter out of an ECL-to-TTL converter chip.

    What matters is stopband rejection. All the classic filter responses try to optimize passband flatness.

    So John doesn't know what he is talking about.

    The jitter of a DDS at low frequencies is domnated by the number of
    MSB bits that we pick from the phase accumulator. It's usually better
    to synthesize a clean octave and divide down as needed.

    And doubles down on being ill-informed.

    --
    Bill Sloman, Sydney

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  • From Lasse Langwadt@21:1/5 to john larkin on Fri Sep 20 00:13:29 2024
    On 9/18/24 00:48, john larkin wrote:
    I can use an Efinix FPGA and a bunch of cheap fast DACs to make some
    DDS clock sources, specifically four. The pain is the lowpass filter.

    Mini-Circuits and other folks make nice surface-mount lowpass filters,
    but they are most all in the GHz range. I want maybe 25 MHz. You'd
    think there would be a market for packaged MHz-range lowpsss filters.

    It's worth pushing the DAC rate as high as possible to simplify the
    lowpass filter. Stay far away from Nyquist.


    https://krfilters.com/products/lowpass/

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  • From john larkin@21:1/5 to All on Fri Sep 20 11:15:05 2024
    On Fri, 20 Sep 2024 00:13:29 +0200, Lasse Langwadt <llc@fonz.dk>
    wrote:

    On 9/18/24 00:48, john larkin wrote:
    I can use an Efinix FPGA and a bunch of cheap fast DACs to make some
    DDS clock sources, specifically four. The pain is the lowpass filter.

    Mini-Circuits and other folks make nice surface-mount lowpass filters,
    but they are most all in the GHz range. I want maybe 25 MHz. You'd
    think there would be a market for packaged MHz-range lowpsss filters.

    It's worth pushing the DAC rate as high as possible to simplify the
    lowpass filter. Stay far away from Nyquist.


    https://krfilters.com/products/lowpass/


    I've asked for pricing on KR 2434-20.

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