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 simulating for sigma delta noise folding due to charge pump nonlinearity 

Last post Mon, Dec 30 2013 2:10 AM by Andrew Beckett. 5 replies.
Started by vamshiky 27 Dec 2013 10:36 AM. Topic has 5 replies and 2178 views
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  • Fri, Dec 27 2013 10:36 AM

    • vamshiky
    • Not Ranked
    • Joined on Tue, Nov 6 2012
    • Bangalore, Karnataka
    • Posts 13
    • Points 230
    simulating for sigma delta noise folding due to charge pump nonlinearity Reply
     Hi,

    In the context of fractional plls, the nonlinearity in the PFD+CP transfer characteristic would result in  noise folding which would increase the low freq phase noise.

    I want to create a simple setup to capture in spectre simulations using
    1.) quantisation noise by using zvcvs ( just by plugging coefficents of NTF of sigma delta modulator(z domain TF))
    2.) vcvs to create the nonlinear transfer char using pwl vector/data file (curve obtained from stand alone chargepump simulation  charge  Vs. delta phi (input phase difference))

    If i cascade these two sources 1,2 can I really capture the folding effects(intermodulation )?
    but what simualtion (ac/tran/pac ) is needed for this kind of situation?


    Ofcourse modeling  modulator/chargepump in timedomain or using matlab for the same is fullblown exercise to capture it completely.

    Thanks,
    Vamshi
    • Post Points: 20
  • Sat, Dec 28 2013 12:01 AM

    Re: simulating for sigma delta noise folding due to charge pump nonlinearity Reply

    Vamshi,

    That should capture the noise folding, although of course the ideal nature of the controlled sources will mean that they are not capturing any device noise of the equivalent blocks in the design - but apart from that, it's fine.

    You cannot use an ac analysis, because this is not a time-invariant linear system. You can't use pss, because if it is a fractional pll (a sigma delta), then there's no periodic solution; so it's a non-periodic time-varying system. Secondly, the z-domain controlled sources also suffer from "hidden states" and so cannot be used with pss.

    So transient is your only solution here, I think (from the level of detail that you've provided, at least).

    Kind Regards,

    Andrew.

    • Post Points: 20
  • Sat, Dec 28 2013 12:44 AM

    • vamshiky
    • Not Ranked
    • Joined on Tue, Nov 6 2012
    • Bangalore, Karnataka
    • Posts 13
    • Points 230
    Re: simulating for sigma delta noise folding due to charge pump nonlinearity Reply
    Thanks for the response Andrew,

    I have planned to simplify the test setup as much possible, using NTF like (1-Z^-1) is sufficient for  modeling quantization noise of sigma delta and I just need to pass it through a nonlinear transfer function. Looks like there is no way to avoid transient.

    Also is there a way to generate a random bit sequence ( for white noise) in transient , so that I can pass it through NTF(z) in spectre?

    Thanks,
    Vamshi
    • Post Points: 20
  • Sat, Dec 28 2013 12:55 AM

    Re: simulating for sigma delta noise folding due to charge pump nonlinearity Reply

    Hi Vamshi,

    You can use vsource in analogLib and set the  type to "prbs". See "spectre -h vsource" for more details. Alternatively there's a veriloga model in (I think) ahdlLib called something like rand_bit_stream. 

    Regards,

    Andrew 

    • Post Points: 5
  • Sat, Dec 28 2013 9:49 AM

    • vamshiky
    • Not Ranked
    • Joined on Tue, Nov 6 2012
    • Bangalore, Karnataka
    • Posts 13
    • Points 230
    Re: simulating for sigma delta noise folding due to charge pump nonlinearity Reply

     Andrew,

    Where can I find this document?

    Application Notes on Direct Time-Domain Noise Analysis using Virtuoso Spectre

     

    Thanks,

    Vamshi

    • Post Points: 20
  • Mon, Dec 30 2013 2:10 AM

    Re: simulating for sigma delta noise folding due to charge pump nonlinearity Reply

    Hi Vamshi,

    Not sure where you saw a reference to that document, but I think that the app note you're requesting has been replaced by this one.

    Regards,

    Andrew.

    • Post Points: 5
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Started by vamshiky at 27 Dec 2013 10:36 AM. Topic has 5 replies.