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PSS Harmonic Balance Analysis, unexpected results

MohsenShah

Hello,

    I am simulating a readout chain with the harmonic balance in PSS analysis. I receive so many warning on imelt and Vgs voltages of transistors but when I see the time-domain signal, there is no such thing. Also, the circuit is very simple (an inverter with the output connected to NMOS, PMOS, or T-Gate switches) and all voltages are withing the range of 0-VDD. I think there is no way I see such crazy voltages (like 6*VDD for the Vgs) but since the warnings are very critical I became obsessed about it. I am using the version IC617. Let me know if you need further information.

PM. The THD results look good.

Parents
  • Andrew Beckett

    Is there a good reason why you're using harmonic balance rather than shooting? For a switching circuit, I would expect shooting is a better choice, as otherwise you're going to have to specify a very large number of harmonics (and it may be insufficient harmonics that is leading to the problem you're seeing).

    Unfortunately you've not really provided enough info (e.g. the PSS settings from the input.scs would be a start, plus the log file which is showing the messages - ideally the entire spectre log file uploaded as an attachment rather than pasted into the post).

    Thanks,

    Andrew.

  • MohsenShah
    in reply to Andrew Beckett

    The shooting goes pretty slow for my circuit because of unknown reasons (can take up to several hours). Besides, for this specific analysis I do not do any switching and just give  DC voltages to the switches; so, the harmonic balance should be fine. With regrard to the information, I wasn't sure where can I find the input.scs file but I took a screenshot from the simulation parameters (hope that's OK) and attached it together with the log file. I simulate with 21 harmonics since it did not add a lot of time to the simulation. FYI, I cannot duplicate the aforementioned warnings while doing transient simulations. Thanks, Andrew!

    P.M. I omitted the Fab info from log file.

    Log.txt

  • Andrew Beckett
    in reply to MohsenShah

    Still hard to debug from this, but I suspect you just need to give some tstab time to allow the circuit to start correctly. Usually this is needed if the circuit is more non-linear. From the log file it doesn't look as if any tstab is running, and the imelt in the first iterations are probably as a result of not having a good starting point.

    So I'd suggest allowing a bit of transient time (via tstab) to ensure the circuit is running OK first.

    Regards,

    Andrew.

Reply
  • Andrew Beckett
    in reply to MohsenShah

    Still hard to debug from this, but I suspect you just need to give some tstab time to allow the circuit to start correctly. Usually this is needed if the circuit is more non-linear. From the log file it doesn't look as if any tstab is running, and the imelt in the first iterations are probably as a result of not having a good starting point.

    So I'd suggest allowing a bit of transient time (via tstab) to ensure the circuit is running OK first.

    Regards,

    Andrew.

Children
  • MohsenShah
    in reply to Andrew Beckett

    Awesome, SOLVED! I just gave 1 us tstab and all the imelt and Vgs overvoltage warnings just vanished.

    Thanks, Andrew Slight smile

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