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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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  • ShawnLogan

    Dear QS,

    The amount of distortion you measure will be a function of both your time domain zero crossings accuracy as well as there record size of the FFT. I am not sure if you have done experiments to determine which is limiting your distortion.

    I might suggest that in lieu of forcing reltol to a small value, you set the value of simulation parameter maxstep to a value that will provide your desired level of time accuracy for the zero-crossings. Otherwise, the maximum integration time step is determined by TSTOP and the value of errpreset - which may cause the time step to exceed the level of accuracy you desire in the time domain simulation.

    Let me know if my thoughts are not clear to you!

    Shawn

Reply
  • ShawnLogan

    Dear QS,

    The amount of distortion you measure will be a function of both your time domain zero crossings accuracy as well as there record size of the FFT. I am not sure if you have done experiments to determine which is limiting your distortion.

    I might suggest that in lieu of forcing reltol to a small value, you set the value of simulation parameter maxstep to a value that will provide your desired level of time accuracy for the zero-crossings. Otherwise, the maximum integration time step is determined by TSTOP and the value of errpreset - which may cause the time step to exceed the level of accuracy you desire in the time domain simulation.

    Let me know if my thoughts are not clear to you!

    Shawn

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