Tip of the Week: Why Do Shooting and Harmonic Balance Phase Noise Results Differ?

Question:

You are simulating your VCO in SpectreRF. 

You ran your PSS + Pnoise (noisetype=sources) simulations using the Shooting engine pss+pnoise and plotted the phase noise. You noticed that the SpectreRF phase noise results differ significantly for the Shooting Newton vs. Flexible Balance engines.   

In the Shooting Pnoise analysis, there are Floquet warnings in your spectre.out file.
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WARNING (SPCRTRF-15093): The Floquet eigenspace computed by the PSS analysis appears  to be ill-defined, so the PNOISE computations might be inaccurate too.

Consider rerunning the simulation with a smaller reltol value, different tstab values, and with method=gear2only'. Check the
circuit for unusual components.
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You tried the above recommendations in the warning message, but there is still a large difference between Shooting and HB results.  Which results are correct?

Answer:

The Floquet warnings indicate that the circuit is ill-conditioned, the simulator is having difficulty finding the correct Floquet eigenvalue, and the results are not correct.  

We have improved Pnoise analysis for autonomous circuits with a new "augmented" mode. Augmented Pnoise will give more accurate results for oscillators with large time constants. In augmented Pnoise, the frequency-aware PPV method (rather than Floquet deflation) is used.  The new method gives a Lorentzian plot.  However, the price you pay is slower simulation speed.
 
To turn on augmented Pnoise, please set the Pnoise option "augmented=yes" for PSS+Pnoise oscillator simulations.

Starting in MMSIM6.2.1 ISR (6.2.1.278), augmented Pnoise is available in HB Pnoise.
Starting in MMSIM7.0.1 FCS, augmented Pnoise is available in Shooting Pnoise.   

If when using Shooting PSS+Pnoise in Spectre versions prior to MMSIM 7.0.1, you see Floquet warnings in the spectre.out file, the workaround is:

    * Use HB PSS+Pnoise and the augmented=yes option.

With the augmented mode enabled, the Shooting PSS+Pnoise should match the HB PSS+Pnoise results, and you will no longer get Floquet warnings.

Note that GUI support for augmented Pnoise is not available in ADE until IC5.1.41 ISR117 and IC6.1.2 ISR13 or later versions.

For more information, please see the online help
spectre -h pnoise