I'm simulating a free running oscillator for jitter and I have the following question:
I have to run a "PNOISE - sources" simulation in order to recieve phase noise, since I have to filter the phase noise before integrating in to extract jitter (in order to mimic a PLL / CDR transfer function).
A few papers were written on the subject, some of them state that the integration upper limit is Fc/2 while others state that it is a few Fc. I assume that it should be a few Fc if the tested wave is a sine wave (i.e. no harmonics appear in the phase noise) and Fc/2 if it is a square wave.
As far as I understand, for square waves the jitter behavior of the first harmonic is similar to the jitter behavior of the square wave, thus it is assumed that integration up to Fc/2 takes into account only the first harmonic, otherwise the jitter will be summed more than once.
Please correct me if so far I'm wrong. Otherwise, here is a correction that I would like to do in my PNOISE simulation settings: instead of mixing the noise with many harmonics (i.e. Maximum sideband >> 1) and then integrating up to Fc/2, I might set maximum sideband to 1, thus the noise will be mixed only with the first harmonic, such that I will see a phase noise as if I had a pure sine wave at the input and not a square wave. Then, I would integrate up to a few Fc and see a more accurate jitter result.
In my simulations I see substantial difference between the two options, that's why the question is very important.
Any respose will we appreciated. I would especially like to hear Andrew Beckett's opinion on this.
I don't have any clear argument why it could be generalized, it just makes sense to me to say so.
Anyway, I can say that (1 - Frank) the dominant noise source is not the oscillator but the stages that extract CMOS clock from the sine wave at the oscillator loop and (2 - Andrew) when I'm not neglecting the higher sidebands and integrate with fullspectrum, range [1K,300*Fc] I see jitter of 23ps so the higher sidebands do make a significant difference.