[time-nuts] Spice simulation of PSRR and phase noise

Gerhard Hoffmann dk4xp at arcor.de
Sun Oct 22 23:00:45 UTC 2017

Am 22.10.2017 um 22:58 schrieb Bruce Griffiths:
> Hoi Attila
> Since close in phase noise can result from up conversion of supply noise etc via circuit non linearities, using an AC analysis won't work.
> Only transient simulation or perhaps analytical modelling of the various non linearities will provide accurate estimates of upconverted PN. If you use transient simulation techniques increasing the level of the various noise sources above the actual levels encountered in real circuits and then correcting the resultant PN back to the level that would be encountered in the actual circuit (using the results of analytical modelling) may be a useful way to reduce simulation time or at least overcome some of the challenges associated with accurately determining low level PN from a simulation.
> There are some in the LTSpice Yahoo group attempting this but they seem way out of touch with the amount of simulation data required. I've provided them with the appropriate formulae to extract PN from the the amplitude spectra. At the moment they appear bogged down with some somewhat trivial peripheral issues.

In a previous life, when I was an EE&CS student, we had to write all the
relevant algorithms ourselves, like building the conductance matrix,
finding the operating point, linearizing nonlinear devices around the
OP, doing the integration over time, companion models etc, b4 we were
given the Spice 2G4 sources...

(Attila, that was a few 100 meters from where you seem to work right
now. There was a beautiful TR440!)

Given that we often enough see convergence problems in integration over
time to the point that the simulator gives up altogether, especially
when there are high Q resonances or nonlinearities around, and that
these errors look like phase noise, I would never ever trust a FFT
result at, say, the -140 dBc level. And there it just starts to be

As much as I like to use LTspice, it's easy availability blocks any fast
progress in the public spices like adding HB, s-params by diverting
people to experiment with add-ons instead of solving the fundamental
issues. X/Ngspice and QUCS are nice but understaffed for sure.

regards, Gerhard.

(who was designing a chopper amplifier in the 140 pV/rt Hz league this
rainy weekend and did not even try to simulate its noise. The
interesting part of it would never make it through the pot core

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