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External clock for Analog to Digital Converter

Discussion in 'Electronic Design' started by East Hunk, Feb 27, 2007.

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  1. East Hunk

    East Hunk Guest

    Hi Everyone,
    I am working on Jitter in Analog to Digital Converters(ADCs). I am
    trying to setup an experiment to see the effects of jitter in real
    time ADCs. I have an ADC evaluation board with external clock input
    for sampling (i.e Sampling clock).
    Now I want to produce a self created jittery signal to see the effects
    of jitter in ADC. Can anyone out there have any idea(s) how I can
    produce a real time clock signal with "variable jitter" for the input
    of external clock? Producing a simple clock signal (i.e. without
    jitter) is straight forward with the help of any signal generator but
    a clock signal with "variable jitter" is a problem.

    Thanks for your help.


  2. Eeyore

    Eeyore Guest

    Add noise. It's an analogue issue effectively.

  3. John  Larkin

    John Larkin Guest

    Get a fast analog comparator or a differential-LVDS-to-CMOS converter

    Apply your low-jitter logic-level signal generator to the + input of
    the comparator. Bias the - input to about logic mid-swing and AC
    couple a noise source into that. The output clocks your adc, and the
    jitter will depend on the slew rate of the edge at the + input and the
    ac noise level at the - input; the math here is direct. The noise
    source can be any waveform you like and will determine the probability
    distribution of the jitter; random, gaussian noise would be the
    typical choice, or use a triangle wave for a flat probability

    To get lots of jitter, you may have to reduce the slew rate of the
    clock generator. Some pulse generators have adjustable slew rate, but
    if yours doesn't, a simple r-l-c lowpass filter can control edge rate
    pretty accurately.

    Some of the adc's around these days need femtosecond RMS clock jitter
    to meet their accuracy specs.

  4. Eeyore

    Eeyore Guest

    Nice detailed explanation there John.

    It's useful to note that most audio converters now simply require wordclock and
    generate the bit clock from an internal PPL.

    As such there's not much you can do to influence their performance.

  5. Lionel

    Lionel Guest

    What sort of clock frequency does your ADC need, how much jitter do
    you want, & how precisely to you need to control it?
  6. Lionel

    Lionel Guest

    It was indeed.
  7. Eeyore

    Eeyore Guest

    Sorry typo ! Yes PLL.

  8. John  Larkin

    John Larkin Guest

    For audio, I wouldn't think that even a microsecond of RMS jitter
    would be audible.

  9. It does, it alters the noise, AFAIR, from a paper from Crystal/Cirrus,
    many years ago, which I cant find, and a AES paper by Julian Dunn,
    1992. Basicallyt, it depends on modulation frequency, and at about 1K,
    needs to be less than 1nS to be inaudible, and less than 100pS as you
    go higher.

    But in these days after a nice linear 24Bit signal has been mangled by
    bean counting software, I suspected 1uS may not be far from the truth

  10. John Larkin

    John Larkin Guest

    I don't understand that. Sound moves about 340 m/s, or 3 milliseconds
    per meter. 3 microseconds per mm. 3 ns per micron. Think about a
    microphone on a stand, sitting on a stage with a thousand people
    walking around, on a street with cars and trucks, in a building with
    people and vending machines and hvac, various musical instruments
    making enough noise to damage your hearing... what's the jitter
    introduced by all that?

    And listening: is your speaker cabinet solid granite, sitting on
    bedrock? Are you sure your woofer has no effect on the position of the
    cone of your midrange? Is your head in a clamp? Do you stop your heart
    and breathing to listen? What do you do about air currents and
    thermals in the room, modulating sound velocity all over the place?

    Picoseconds are absurd here.

  11. On Tue, 27 Feb 2007 11:07:13 -0800, in John
    Not really, I think that you are just looking at it from the wrong
    I've dumped a couple of papers:
    the Steven Harris, should be on the cirrus site, but it's down for

  12. John  Larkin

    John Larkin Guest

    Silly. One of the things he doesn't note is that adc or dac clock
    jitter doesn't create signal energy, it just splatters signal around
    the spectrum. So 8 ns of jitter may raise the noise floor to, say, 84
    dB below a 0 dB signal, but the jitter adds no noise when there's no
    signal. Who can hear -84 dB of noise *when*the*main*signal*is*0*dB ?

    Jitter doesn't add to the "quiet" background, which is the one that
    might conceivably be audible. As long as nobody in your listening room
    is, say, breathing.
    I can't load that one for some reason. I'm guessing it must be equally

  13. Not many people record at FS, normally there is 12dB headroom, thats
    standard practice, so that's now down to -72dB, and if you are mixing
    many sources the noise just tends to add up, so 8nS is not

    Sorry about the second file, try this
    I've been trying to find ref(8), but its's not freely available on

  14. Eeyore

    Eeyore Guest

    Fancy that. I've worked with Julian in the past.

  15. East Hunk

    East Hunk Guest

    Hi John,

    Thank you for your reply. This is indeed an excellent idea. Let me try
    this and then I will get back to you.


  16. Ian

    Ian Guest

    I see that's a preprint, did the paper ever get into the Journal?

  17. John  Larkin

    John Larkin Guest

    No. This noise is a spectral scattering of the main signal. As the
    program level declines, the jitter-induced noise floor tracks it down.
    If the jitter-caused noise is 84 dB below 0 dBm, it's also 84 dB below
    -40 dBm, namely -124. No signal, no noise.

    This isn't like hum or thermal noise; they are constant adders to the
    signal. This is more like distortion, a *multiplier*.

    The only time moderate sampling jitter introduces noise is when you
    can't hear it.

  18. Thanks for that explanation, I see what you mean now

  19. No idea, gave up the AES mag years ago, it was much was too esoteric,
    I spent all my time keeping 6 sound studios + all the other bits
    running. Might be worth a search through AES site

  20. joseph2k

    joseph2k Guest

    I really doubt the jitter is that bad, 10% of (clock) conversion rate in
    jitter is a really atrocious clock. Few reasonably appropriate clock
    generetors have over 0.01% jitter, which would be about 1 ns for audio
    work. Soounds like one of the differences between home audio and pro
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