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Simulators >> RF Simulators >> PSS noise simulation using ahdl models
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Message started by Walt on Jul 9^th, 2003, 7:42am

Title: PSS noise simulation using ahdl models
Post by Walt on Jul 9^th, 2003, 7:42am

I'm trying to model a simple PM modulator corrupted by noise. The SpectreRF test bench uses an ahdl/verilog model (ahdlLib/pm_modulator) whose modulation input is driven by a noisy source (i.e. RC low-pass filtered thermal noise from a resistor and a vcvs gain block). While PSS & pnoise do not complain, I wonder if these ahdl models can properly handle a noise simulation.

Can anyone offer some comments on this approach?

Walt

Title: Re: PSS noise simulation using ahdl models
Post by Ken Kundert on Aug 17^th, 2003, 10:44pm

When using Verilog-A models with SpectreRF there are two things to worry about. First is hidden state, which is discussed in the paper that can be found at http://www.designers-guide.com/Analysis/. If there is hidden state, then the RF analyses generally just refuse to run, or if they do run, produce convergence problems. If your models are running without difficulty, then hidden state is generally not a problem.

The other problem that might occur is present only with small signal analyses, such as AC, noise, PAC, PNoise, etc. It occurs if the model contains an abrupt discontinuity, such as might be the case if one were modeling an ideal comparator. This is a conceptual problem more than anything else. Consider a model that produces 1 if the input is positive, and -1 if the input is negative. A small signal analysis will indicate that none of the small input signal will pass through the comparator (its gain is zero) because the input signal, being small, is too small (by definition) to trigger the comparator. Thus, no noise (or jitter) will pass through the comparator. This is not really correct because 1) no comparator is ideal, and 2) when it comes to comparators, no signals are small if the input is biased at the threshold comparator. If your model does not contain abrupt discontinuities (the actual function is discontinuous, not just the derivatives), then this one should not be a problem.

If you'd like to know for sure, post your model and I'll try to take a look at it.

-Ken

Title: Re: PSS noise simulation using ahdl models
Post by Mark Gehring on Sep 10^th, 2003, 11:34am

Another thing to look out for is noiseless resistors inside the AHDL model. You will need to add noise if you are simulating resistance in that model. I assume that the PNOISE analysis will use it, but I have not tested that yet. For example, we have a MOS varactor model with nonlinear resistance modeled inside the AHDL code. We recently realized that noise from that resistance will not be in the simulation - only its de-Q'ing effect will be seen.

Title: Re: PSS noise simulation using ahdl models
Post by Andrew Beckett on Sep 10^th, 2003, 2:11pm

Provided that you use the Verilog-A functions for adding
noise sources, then that noise will be included in any of the
noise analyses - noise, pnoise, qpnoise, sp, psp, qpsp.

It won't add noise just because you have the equation of
a resistor, because it doesn't really know that it is a
resistor - you might be modelling something which has
an ideal noiseless impedance.

Regards,

Andrew.