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
interesting.
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
transformer.)
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