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half harmonic frequency in PSS analysis

Andy Liu
I am designing an LC based voltage control oscillator. I want to simulate phase noise of my oscillator, but, I ALWAYS got a very bad phase noise performance (for example, -60 dBc/Hz @ 1 MHz offset, compared to my expectation -110 dBc/Hz). The reason is that in the periodic steady state analysis (PSS), the fundamental frequency is simulated wrong. I estimate the fundamental frequency is 5 GHz from the transient waveform, whilst the fundamental frequency given by PSS simulation is 2.5 GHz. Has anyone encountered or does anyone have any knowledge of this problem?
 
I figure out a method to get around this half harmonic frequency problem. In the pnoise analysis, people usually choose “sweep type” as “relative”, and “relative harmonic” as 1. I wonder if I can choose “relative harmonic” as 2 to compensate the half harmonic in PSS simulation.
 
Thank you.
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  • Andrew Beckett

    Look at the end of the harmonic balance simulation - you'll see that the frequency matches that used in the pnoise. The whole point of the frequency printed out at the beginning of the PSS is that it is an estimated frequency. It's used to help the harmonic balance start with a reasonably close estimate of the actual settled frequency - the harmonic balance iterations will take care of converging on the final, settled, steady state frequency - which is  6.4471e+09 as shown in the 47th (i.e. last) iteration of the harmonic balance. Since it seems to be taking a while to converge, I'm wondering whether you have enough harmonics, or whether you need a bit more tstab time to help things along. But either way, there is no consistency between the PSS and pnoise analyses.

    Andrew.

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  • Andrew Beckett

    Look at the end of the harmonic balance simulation - you'll see that the frequency matches that used in the pnoise. The whole point of the frequency printed out at the beginning of the PSS is that it is an estimated frequency. It's used to help the harmonic balance start with a reasonably close estimate of the actual settled frequency - the harmonic balance iterations will take care of converging on the final, settled, steady state frequency - which is  6.4471e+09 as shown in the 47th (i.e. last) iteration of the harmonic balance. Since it seems to be taking a while to converge, I'm wondering whether you have enough harmonics, or whether you need a bit more tstab time to help things along. But either way, there is no consistency between the PSS and pnoise analyses.

    Andrew.

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