PSS simulation

OK, I've set something simple up to show the problem you're seeing - in order to illustrate it. I have this as a netlist:

//
parameters fsamp=27.918M

vpwl (pwl 0) vsource type=pwl file="clock.pwl"
vsample (sample 0) vsource type=pulse period=1/fsamp width=0.5/fsamp 
+ rise=1n fall=1n val0=0 val1=1

rsamp (sample sampout) resistor r=100
csamp (sampout 0) capacitor c=0.1p

pss pss fund=fsamp tstab=72.2914n

pnoise (sampout 0) pnoise start=10 stop=100 dec=5 maxsideband=10 
+ noisetype=timedomain numberofpoints=0 noisetimepoints=[35n] annotate=status

And then this as my clock.pwl file:

0 0
10n 2
20n 2
30n 0
106n 0
107n 2
108n 2
109n 0
111.55n 0

In order to get the numbers you're seeing, the last point in the pwl file must be at 111.55n not 198n.

Don't worry at this stage that the clock.pwl file is not representing your 1.2GHz clock (I was just being lazy when creating this file; it's suffiicient to explain what is going on). The first part of the PSS simulation will start at time 0, and then go to the point at which all the sources become periodic (111.55n, because the pwl source is not seen as periodic), and then simulate any tstab time you've set (72.2914n) and also 1 period of the PSS fundamental (1/27.918M = 35.819n). So that's a total of 219.661ns (which is what you see). Each PSS iteration will then re-simulate the last 35.819n of this over and over again (i.e. from 183.841 ns -> 219.661 ns).

The timedomain noise sample you have asked for is at 35ns through this 35.819ns period, so therefore will be 0.819ns from the end of the 219.661n section (i.e. 218.841ns).

So the real problem is that your pwl file is not being treated as periodic - understandably, because the simulator had no idea it was meant to be periodic and not a one-off reset pulse (say).

In order to make it periodic, you need to specify the pwlperiod parameter (which on the analogLib vpwlf source shows up as  "Period of the PWL", or on the analogLib vsource component in pwl mode shows up as "Period" (there's also "Period Start Time" which corresponds to the pwlperiodstart parameter I mentioned earlier).

Having made this periodic (I suggest running a long transient just with this pwl source just to make sure it's repeating OK), the problem is that your PSS fundamental will need be commensurate with both the 27.918MHz sampling signal and your 1.2GHz clock. Unfortunately the greatest common divisor of both those frequencies is 6kHz (think about it - to do a Fourier analysis, you're going to need whole numbers of both frequencies). Since that corresponds to 200000 cycles of the clock signal, your simulation would be very slow, and will need an enormous amount of memory.

There are several choices:

1. Adjust your frequencies a little to make them commensurate (or more commensurate). For example, a Fsample of 28MHz would mean a common frequency of 4MHz. That's still 300 cycles of the clock, but may be manageable.
2. Check to see if the clock (or the sampling signal) really needs to be there to check out the noise performance. Dealing with a single frequency makes the simulation much simpler
3. You could potentially use QPSS (Quasi-periodic steady state) with the two frequencies, but the trouble is that two highly non-linear signals is very expensive to simulate. QPSS (shooting) is intended primarily for when you have one strongly non-linear, and the rest weakly non-linear signals. With Harmonic Balance, all signals are typically weakly non-linear, but you could ask for lots of harmonics of one of them; asking for lots of harmonics for multiple signals is likely to lead to very slow and memory hungry simulation time though.

Unfortunately it's hard for me to be precise because I don't quite know what the logic you're describing is doing, and would probably need to see it. It might be easier to talk to Cadence Customer Support so that we can see more of the data and give you more focused advice?

Best Regards,

Andrew.

Excuse me Mr.Beckett, I and my friends are running a Virtuoso 6.1.5 and are wondering how to set the stop time in PSS simulation properly, could you please tell me? I will be really appreciated that! : )

Excuse me Mr.Beckett, I and my friends are running a Virtuoso 6.1.5 and are wondering how to set the stop time in PSS simulation properly, could you please tell me? I will be really appreciated that! : )

I don't really know what you mean by stop time. There is no stop time in PSS...

Perhaps you should post a new question (the forum guidelines ask you not to post in old threads anyway) with details of what you're trying to do and then I or somebody else can give a more focused answer?

Regards,

Andrew.