sideband in PNOISE vs harmonic in PSS

StanleyChe said:

Second of all, I find it inconsistent that in PSS spectrum plotting, I can only see up to 4 harmonics but in PNOISE, it is then ok to see noise output translated to 6th harmonic (simply because PSS saves enough data, up to 40*fund?). Doesn't that confuse the user?

No, it's not weird and shouldn't be confusing (although it's clearly confusing you). As I keep saying, the number of harmonics in shooting PSS is not an accuracy control - it's to control the number of output harmonics for plotting purposes. For some kinds of circuit (e.g. a switch-cap filter) plotting the harmonics of the clock at various points in the circuit is not very interesting, but the PSS time-domain solution is needed to allow small-signal analyses (e.g pac, pxf, pnoise). This is all because it's a time-domain based approach and the solution is found and represented in the time-domain.

This is a key difference from harmonic balance where the solution is found and represented in the frequency-domain. In that case the time-domain has to be generated from the frequency-domain with an inverse Fourier transform, and because the frequency-domain solution is used as the basis of the small-signal analyses, the harmonics of the hb do matter for the small-signal analyses.

We always used to cover this (at least I did) as part of teaching Spectre RF - it may be covered in the current (free) online course material on Cadence Online Support.

It is actually described in the SpectreRF Theory chapter for PSS too. It says:

Simulation Accuracy Parameters

The accuracy of your simulation results depends on your accuracy parameter settings, not on
the number of harmonics you request. It is recommended that you adjust the accuracy
parameters using the errpreset parameter settings as described in “The errpreset
Parameter in PSS Analysis” on page 191.

A little later it says:

The maxacfreq parameter automatically adjusts maxstep to reduce errors due to aliasing in
subsequent periodic small-signal analyses. By default, maxacfreq is four times the
frequency of the largest harmonic you request, but it is never less than 40 times the
fundamental.

Andrew

I think I see your point after reading your responses several times again. Gotta look PSS + PNOISE in shooting method literally as a time domain operation, then fourier to present freq domain data. Result is not obtained through convolution in freq domain. What was so difficult is that all the PSS/PNOISE terms used in the setup are literally freq domain terms: harmonic, sideband, relative harmonic, maxacfreq, etc, which makes people think the underlying engine is operation in freq domain. Even in the spectreRF manual, the freq translations plots are all explained from freq domain point of view.