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zvcvs parameters for S&H, in ac, implementation

agaurav

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. 

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  • Andrew Beckett

    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.

Reply
  • Andrew Beckett

    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.

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