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PLL and SYNC question

Discussion in 'Electronic Design' started by kirgizz, Dec 9, 2005.

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  1. kirgizz

    kirgizz Guest


    I'm dealing with synchronisation issue, that is a big problem for me.
    I read some ebooks about PLL so I understand (theoretically) it

    My DSP must be able to generate (internally) a sine wave which is
    frequency-locked to an external TTL reference signal with an error
    about 0.01Hz (at 20kHz). The frequency range of the external signal i
    20Hz-20kHz. I use the DSP in place of VCO, but it doesn't work proper.

    More detailed:
    ext. TTL signal -> PLL SIGin -> PFD -> Loop Filter -> ADC -> DSP ->
    DAC -> Comparator -> PLL COMPin.

    My problems are
    1. External signal is variable (and I can't get some error-linearit
    loop filter for the whole freq-range).
    2. All commercial PLLs (such xx4046 and xx9046) are designed fo
    MHz-range, and not for my little 20kHz.

    Settling time is depending on the sampling frequency (not so good too?).

    I read about software PLL and I think that it's too complex to implemen
    for frequencies up to 20 kHz.
    ADPLLs...hmmm, I can't imagine, how I use it form my design (I mea
    available xx297 PLLs), but I'd rather do all digitally.

    Can you help me with an advice how I do this synchronisation?
    Maybe there are another ways to do it.

    Thank you in advance
  2. Tim Wescott

    Tim Wescott Guest

    What do you mean by error-linearity?!?
    Huh? 4046's only work into the low MHz; with big enough capacitors
    they'll work down to the single-digit Hz.
    This is just something you'll have to live with.
    I've implemented software PLLs up to 80kHz on modern DSP chips. You
    need to have a fast DSP to be able to update the control loop at that
    rate, or you need to figure out how to subsample the input without
    messing up. If you can generate a sine wave at 20kHz in your DSP you
    must be turning a routine over at over 40kHz, so I assume you have a
    fast DSP.

    The key is to use a timer for the phase detector. If you have a timer
    capture input you can capture the timer's phase at the instant that the
    input signal happens. If you use the same clock that's generating your
    sine wave for input capture you can easily relate the input phase to
    your output phase.

    Yes, your loop will be more complex. But with this complexity you can
    buy easy linearization, gain scheduling to account for the changing
    sampling rate as your input rate gets low, and good fast rough frequency
    estimation to start your NCO at the right frequency and phase for quick
  3. Tim Shoppa

    Tim Shoppa Guest

    My DSP must be able to generate (internally) a
    "Frequency error" is a useless spec. When a PLL is locked, there is no
    frequency error. But there is phase noise, spurs, etc.
    Not true, 4046's work great at that frequency range and much lower too.

    I suspect your main problem is specifying what you want. If you tell us
    what comes in and what goes out, there's probably an easy solution.

    If it really is a 20kHz signal in (TTL? Square wave?) and you want a
    20kHz sine wave out
    there are much easier solutions than a software DSP PLL.

  4. Guest

    Have a look at an Analog Devices DDS chip - they started off with the
    AD9850 but they now have a page of so of them on their web-site
    covering a wide range of prices and speeds.

    That could be your software controlled oscillator, and would do 20kHz
    very nicely - the frequency resolution is about 0.001Hz and you can do
    phase increments and decrements as well as very fine frequency
    adjustments, which should get all the busy stuff out of your DSP . If
    you wnat to get fancy and set up in-phase and quadrature phase
    detectors, Analog do a couple of DDS chips that synthesise both
    in-phase and quadrautre outputs.
  5. kirgizz

    kirgizz Guest

    to Tim Wescott

    Thank you for the quick answer.
    I didn't mean the phase comparator output but transient phase
    error signal after the loop filter (that must be fed to the VCOin).
    I implemented 1st order passive low pass filter as the loop filter.
    You know in this case you have not the same signal forms if you have
    100 Hz or 20kHz loop input.

    I agree. But I don't use VCO.

    Really? Great performance (I'd like to have too).
    My DSP is SHARC ADSP21262 running at 200 MHz. I think it's suitable. But
    there are another algorithms to be implemented in my design (filters).

    I used to capture period of the TTL-input by the timer and to generate
    corresponding sine wave. This worked very well for 100-Hz but was not
    precise for 20KHz input. Did you use 1 timer for the phase detector?
    Could you describe the whole process more detailed, please?

    Yes, I'd like to implement a software PLL and escape using additiona
    hardware. Did your implemented SPLL worked for defined frequency only
    or for a frequency range, how large was the lock range? My software must
    be able to lock to frequencies from 20Hz to 20kHz.

  6. kirgizz

    kirgizz Guest

    to Tim Shoppa

    Thank you.

    I agree.
    In order to implement an lock-in amp on DSP I want to generate
    quadrature sine waves internally. And they must be locked to an externa
    reference signal.
    TTL reference goes in, frequency range is 20Hz-20kHz. Reference i
    variable and ref.frequency sweep is possible.

  7. kirgizz

    kirgizz Guest

    Thank you Bill
    Yes, I accept this DDS chips as an option. However, I'd rather do i
    any additional hardware. Of course, I can preciselly set th
    phase/frequency by DDS.
    In my design DDS-output must be locked to the external reference frequenc
    so I need some sort of lock-detection and phase comparator too. The mai
    problem of synchronisation is not solved. (Or am I wrong?)

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