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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

 

Parents
  • ShawnLogan
    Dear Andrew and QS,

    I apologize for my potential suggestion to utilize maxstep in your application QS. As most of my simulations are focused on various types of oscillators, I set this option frequently to provide the level of measurement accuracy I need.

    My apologies again for any confusion I caused.

    Shawn
Reply
  • ShawnLogan
    Dear Andrew and QS,

    I apologize for my potential suggestion to utilize maxstep in your application QS. As most of my simulations are focused on various types of oscillators, I set this option frequently to provide the level of measurement accuracy I need.

    My apologies again for any confusion I caused.

    Shawn
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