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 zvcvs parameters for S&H, in ac, implementation 

Last post Wed, May 2 2012 5:24 AM by Andrew Beckett. 8 replies.
Started by agaurav 25 Apr 2012 06:44 AM. Topic has 8 replies and 3567 views
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  • Wed, Apr 25 2012 6:44 AM

    • agaurav
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    zvcvs parameters for S&H, in ac, implementation Reply

    Hi,

    I want to make an ac model which has got sample and hold (zero-order). I found the block, zvcvs, for that. Can somebody suggest the suitable parameters' values to implement that.

    I tried Polynomial argument = z or inversez

    S to Z Transformation = default (I assume it means none)

    Specification type = polynomial

    And then I tried various polynomails (order 1 or 2) to get sample-and-hold fft output for a simple wideband spectrum signal-input but seems I am doing something somewhere wrong. I am not very good at digital and z and bilinear transforms, so I think the capability is there but somehow I am not able to implement it using zvcvs. Can somebody please help me with it - implementing sample and zero-order hold using zvcvs. 

    • Post Points: 20
  • Thu, Apr 26 2012 5:38 AM

    • keble6
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    Re: zvcvs parameters for S&H, in ac, implementation Reply

     That's interesting - I am trying to use a zvcvs in a PLL model and it doesn't do what I assumed it would do!

    I hope someone will be along soon ...

    • Post Points: 20
  • Tue, May 1 2012 8:40 AM

    • agaurav
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    Re: zvcvs parameters for S&H, in ac, implementation Reply
    Can anybody please give some feedbacks?
    • Post Points: 20
  • Tue, May 1 2012 10:15 AM

    • Tawna
    • Top 25 Contributor
    • Joined on Fri, Jul 11 2008
    • Snohomish, WA
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    Re: zvcvs parameters for S&H, in ac, implementation Reply

     Hi Agaurav,

     

    If you are not getting an answer on the forum, I recommend contacting Cadence Customer Support at http://support.cadence.com and filing a Service Request.

     

    Best regards,

    Tawna

     

    Best regards, Tawna Wilsey Staff Support AE, Global Customer Support Cadence Design Systems, Inc.
    • Post Points: 20
  • Tue, May 1 2012 10:53 AM

    Re: zvcvs parameters for S&H, in ac, implementation Reply

    You can do an implementation of 1/(1-z^-1) by using:

    //
    vsin (samp 0) vsource type=sine ampl=1 freq=20k
    r1 (samp 0) resistor r=1k
    sh (hold 0 samp 0) zvcvs ts=1u numer=[1] denom=[1 -1]
    r2 (hold 0) resistor r=1k

    tran tran stop=3/20k

    For example. Note that you can't simulate this in an "ac" analysis - this doesn't make any sense. And you can't simulate this with pss/pac (which might make some sense because it has a periodic operating point) because the z-domain controlled sources are not supported in the Shooting Newton method because they have "hidden states". It's not clear to me if that's what you wanted anyway...

    Andrew.

    • Post Points: 20
  • Wed, May 2 2012 1:50 AM

    • agaurav
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    Re: zvcvs parameters for S&H, in ac, implementation Reply
    Hi Andrew, Thanks for your response. As usual, with many cases in past, you have come to the rescue. What I am looking for is a block which outputs the fourier-transform of the signal when the signal goes through that sample and hold circuit with clock frequency as the input parameter to the S&H block., in AC noise simulations. I have already tried what you suggested in zvcvs but the response doesn't seems to be in alignment with my expectation. If this block is not supposed to work in AC simulations, I'll expect it to flag and error, not some incorrect result. So, probably, it works in AC sims. But I am not sure if I want an output of signal G(s) going through H(z) and then look at its AC output (say some F(s)), should I select none/forward/backword/bilenar for S to Z Transformation parameter input. I have tried few zero-order hold possible options (I want to implement Zero-order hold by H(z) which I want to use zvcvs for) like z+1/z-1 and simple 1/z but noe of them give output as I desire/expect. I just want to know where am I going wrong. A bit more detailed document about this block will be extremely helpful I also had another problem of generating a noise source with logarithmic interpolation which I found a solution to using noise-source with 2 frequency-noise paris and passing ti through a svcvs which has pole-zero as specification type. So, this is the last bit I need to complete my model I am trying to build.
    • Post Points: 20
  • Wed, May 2 2012 5:01 AM

    Re: zvcvs parameters for S&H, in ac, implementation Reply

    Both ac and noise analyses perform a small-signal analysis around a DC operating point. For any circuit with periodic behaviour, simulating around a DC operating point is not terribly useful - it's not wrong, but almost certainly not what you're looking for. For example, if you have a switch capacitor filter, simulating the noise around a particular bias point, with some of the switches open and some closed will not tell you anything very useful - because you really need the time-averaged response, not the response about a single DC operating point. That's what pss/pac or pss/pnoise give you - the pss captures a periodic steady state (i.e. a periodic operating point) and then the small signal analysis gives you a time-averaged small-signal response over that period.

    The same is true if you use a zvcvs - a single bias point is meaningless. It shouldn't error out - in the same way as it shouldn't error out if you try to analyse a circuit which relies on periodic behaviour in ac/noise. It does however error out if you try to analyse it using pss because the component has some internal "state" storage which is not visible to the PSS solver. The way you'd have to solve that is by using a suitable Verilog-A model as outlined in this paper on Hidden States.

    Regards,

    Andrew.

    • Post Points: 20
  • Wed, May 2 2012 5:12 AM

    • agaurav
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    Re: zvcvs parameters for S&H, in ac, implementation Reply
    You are right Andrew. But all I am trying to do is noise-shaping using transfer-functions from various points in a network, which one can do in matlab but one has to derive the transfer-function from various noise-input points mathematically i.e. derive various F(s) etc. which becomes quite complicated if you have 3 or higher order. Cadence can solve that if I can directly put the network of R and C and other things using the standard components and it can then derive the transfer fucntion for me from various sources to the output without me having to do that exercise mathematically. So, I already have the input-noise-shape etc. defined for the DC operating point I am working at. Only the transfer function from various points, I want to derive. I hope I am able to explain.
    • Post Points: 20
  • Wed, May 2 2012 5:24 AM

    Re: zvcvs parameters for S&H, in ac, implementation Reply

    I'm not sure what your question is from the above? I've explained why what you're trying to do doesn't make sense with a zvcvs.

    If you had a laplace representation you could use svcvs, in which case all would be OK (because these do not require periodic behaviour). Maybe that's what you want? But if you really want a sample-and-hold, you'll have to use a Verilog-A based approach and use PSS and corresponding small-signal analyses. Or use the real circuit (and PSS)

    Andrew.

    • Post Points: 5
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Started by agaurav at 25 Apr 2012 06:44 AM. Topic has 8 replies.