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Spectre setting for comparator-based PWM

greatqs

All,

I'm simulating a circuit with sinusoid being open-loop PWM modulated (natural sampled). Theoretically, the open-loop PWM modulation should be linear (no direct in-band signal harmonics). However, when I did the Spectre simulation with sweeping the reltol parameter, I found the distortion after anti-aliasing filtered and FFT is very big ( < 80dB) highly dependent on reltol with significant numerical noise floor. The THD gets flat (~140dB) until I set 'reltol' to some ridiculous value (method=gear2only, errpreset=conservative, reltol=1e-9).

I suspect this relates to Spectre time step algorithm to handle sharp zero-crossing points with good enough time step "accuracy" as I observed "numerical" jitter before in some frequency divider design which can be workaround by time step algorithm (i.e. LVLTIM) in other simulator.

Does anyone has experience on this and could share some lights?

Thanks,

QS

 

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  • Andrew Beckett

    Shawn,

    No need to apologise - I'm very glad that you provide help to others on the forums. We all (including me) have things we can learn now and again!

    Note that the primary reason I use maxstep with oscillators is to help ensure enough points for the oscillator to actually start properly. I would probably use the PSS or HB analysis to analyse oscillators with sufficient accuracy (shooting PSS also controls maxstep in the shooting interval, but that is mainly to ensure spectral accuracy at high harmonics of the PSS fundamental). I don't normally use maxstep to control accuracy of the oscillator though - I'd use the accuracy parameters for that. But hey, your mileage may vary ;-)

    Kind Regards,

    Andrew.

Reply
  • Andrew Beckett

    Shawn,

    No need to apologise - I'm very glad that you provide help to others on the forums. We all (including me) have things we can learn now and again!

    Note that the primary reason I use maxstep with oscillators is to help ensure enough points for the oscillator to actually start properly. I would probably use the PSS or HB analysis to analyse oscillators with sufficient accuracy (shooting PSS also controls maxstep in the shooting interval, but that is mainly to ensure spectral accuracy at high harmonics of the PSS fundamental). I don't normally use maxstep to control accuracy of the oscillator though - I'd use the accuracy parameters for that. But hey, your mileage may vary ;-)

    Kind Regards,

    Andrew.

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