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 think that your simulation results can be generalized. For your circuit, oscillator phase noise is probably the dominant noise source. In this case, the jitter of every sideband is the same. This is generally not the case for jitter due to other noise sources.
If I have to simulate jitter, I always use pnoise PM jitter analysis. It's simply the most direct way to do it, which reduces the risk of making an error in the simulation setup.