I'm simulating the phase noise of an LC-VCO. Sometimes I get the following warning:

PSS analysis may be insufficiently accurate for PNOISE analysis. Consider tightening simulation tolerance.

I have tried reducing reltol, vabstol, and iabstol to much smaller values than recommended by the manual but nothing changed.

I searched previous threads but in the only relevant one the issue was sweeping frequencies all the way down to zero thus creating flicker-noise related issues. I'm only looking at above 1kHz and I have turned off all flicker noise sources of active devices.

Amy ideas on how to get rid of these warnings?

Thanks,

Thanks for your response.

I'm using Spectre so I run PSS followed by PNOISE to simulate phase noise.

I believe because I'm using Spectre PSS, the solver is Shooting_Newton. The parameters you have referred to are calculated automatically by Spectre based on analog options.

In terms of maximum number of sidebands and also tstab I believe they are large enough because further increase in them does not change anything.

Here is my simulation command:

simulatorOptions options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 temp=27 \
   tnom=27 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 maxnotes=5 maxwarns=5 \
   digits=5 cols=80 pivrel=1e-3 ckptclock=1800 \
   sensfile="../psf/sens.output" checklimitdest=psf
dcOp dc write="spectre.dc" maxiters=150 maxsteps=10000 annotate=status
dcOpInfo info what=oppoint where=rawfile
pss  (  Out  Outb  )  pss  fund=18G  harms=80
+    errpreset=conservative  tstab=100n  skipdc=no  method=gear2only
+    tstabmethod=gear2only  annotate=status
pnoise  (  Out  Outb  )  pnoise  relharmnum=1  start=1k
+       stop=100M  dec=20  maxsideband=80  annotate=status
modelParameter info what=models where=rawfile
element info what=inst where=rawfile
outputParameter info what=output where=rawfile
designParamVals info what=parameters where=rawfile
primitives info what=primitives where=rawfile
subckts info what=subckts  where=rawfile
saveOptions options save=selected pwr=none

Here is the log file. I removed part of the preamble for confidentiality reasons. There are many breakdown warnings, which I'm sure are not the reason for PNOISE failure (I have other simulations without such warnings and yet they give the warning regarding PNOISE accuracy).

The PSS command used in this simulation is:
simulatorOptions options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 temp=27 \
   tnom=27 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 maxnotes=5 maxwarns=5 \
   digits=5 cols=80 pivrel=1e-3 ckptclock=1800 \
   sensfile="../psf/sens.output" checklimitdest=psf
pss  (  Out  Outb  )  pss  fund=15G  harms=80
+    errpreset=conservative  tstab=100n  skipdc=no  method=gear2only
+    tstabmethod=gear2only  annotate=status  tstart=20n
pnoise  (  Out  Outb  )  pnoise  relharmnum=1  start=10k
+       stop=100M  dec=20  maxsideband=80  annotate=status

I would assume some sort of sweep is being done (perhaps via the parametric analysis tool?) which could explain why the netlist shows a single analysis,  but the log has several.

For a 15GHz (ish) oscillator, 80 harmonics seems rather excessive. Perhaps that's why it is struggling with accuracy - it thinks that the PSS solution is not accurate enough at higher harmonics (I'm guessing here).

What kind of oscillator is it - that would govern the choice of HB or shooting? That said, you're not going to have massing amounts of harmonic content if it's a 15GHz oscillator, so shooting is unlikely to give you much advantage.

Regards,

Andrew.