Hi all,

I want to simulate the frequency reponse of an amplifier having a small non-linearity. Hence, the operating point depends on the input's amplitude. The shift in the operating point in turn changes the gain of a stage. This shifted gain in turn causes a move of the resonant frequency. This can be analyzed with multiple transient simulations folled by a fourier transform.
However a PAC analysis should yield the same information with much less effort. I use a vsin-source as stimulus with "PAC magnitude = 1". I vary the amplitude of this source as necessary but I have no clue what frequency to choose. The result of the PAC analysis changes when changing the frequency.
Does someone has an idea or can recommend a good book or paper on PSS and PAC, I am quite new to these analyses but found no good information source yet.
Thanks, achim

Hi.

You should sweep large signal amplitude in Master PSS analysis, then do PAC analysis with pacmag=1.

Here is very simple example.

vin (in 0) vsource type=sine freq=fin ampl=amp pacmag=1 fundname="Fin"

sweeppss  sweep  param=amp  start=1  stop=10  step=1 {
 pss  pss  fund=100M  harms=8  method=gear2only  annotate=status
 pac  pac  start=10  stop=50k  dec=200 maxsideband=0  annotate=status
+      freqaxis=absout  save=allpub
}

[quote author=pancho_hideboo  link=1126179766/0#2 date=1126192699]Hi.

You should sweep large signal amplitude in Master PSS analysis, then do PAC analysis with pacmag=1.

Here is very simple example.

vin (in 0) vsource type=sine freq=fin ampl=amp pacmag=1 fundname="Fin"

sweeppss  sweep  param=amp  start=1  stop=10  step=1 {
 pss  pss  fund=100M  harms=8  method=gear2only  annotate=status
 pac  pac  start=10  stop=50k  dec=200 maxsideband=0  annotate=status
+      freqaxis=absout  save=allpub
}
[/quote]

This is exactly what I have done so far.
You have set the soucre's frequency freq=fin -> what value to choose?
You have set pss-fund=100MHz but the pac-sweep from 10Hz to 50kHz, why those values?
- achim

Hi Ken,

I tried to use the swept PSS, but I suppose I did not have understood what is going on during a PSS: I tried to use a very simple circuit: an ideal resistor, an ideal capacitor and an ideal inductor with a series resistor that are connected in parallel.

V0 (net1 0) vsource dc=5 mag=1 type=sine ampl=1 freq=frq
L0 (net1 net8) inductor l=18m
C0 (net1 0) capacitor c=10p
R1 (net8 0) resistor r=1
R0 (net1 0) resistor r=100K
ac ac start=10k stop=10M annotate=status
sweeppss  sweep  param=frq  start=10k  stop=10M  dec=20  {
 pss  pss  fund=frq  harms=0  errpreset=moderate  annotate=status

 pac  pac  sweeptype=relative  start=frq  maxsideband=0
+      annotate=status
}

now I've done both: an AC-analysis and a swept PSS and measured the current flowing out V0. In AC-simulation the current is, as expected at minimum at about 280kHz.
The PSS results in an output current of 5A which is comparable to the DC-operating point. As I am interested in the AC amplitude is run a subsequent PAC.
For PAC I've ploted the source's current, as sweep I've used "frequency" and as signal level I' used peak. In this case the results of the AC and the swept PSS/PAC analysis are identically. However, if I use "variable" as sweep value, the resonant frequency is to be found at a different location. Any explanations for this behaviour?

Thanks in advance,
- achim