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.
Thanks Yizh for your reply, I appreciate your interaction, I wish we
don't walk in circles but converge as we progress. Below a quick reply
to your questions.1. Yes this is because the basic principle
behind periodic noise analysis is that all harmonic components in the
large signal simultaneously cooperate to shape noise in any bandwidth.
So you may not accurately reproduce this behavior saying I neglect the
harmonics cooperation and consider just the fundamental one. 2. Are you referring to the maxim application note ? it basically says that for an oscillator, jitter measured from filtered fundamental harmonic is practically the same as jitter of the overall square wave output. As I already said this is true, provided your oscillator is pretty clean, i.e. no significant frequency conversion noise (clean output buffer).
3. For a driven circuit, zero crossing jitter for a square is zero, because jitter is simply j = noise/slew_rate ; and slew_rate = dv/dt is infinite at zero crossing for an ideal square.
4. Two cases for square wave:
- Driven circuit: do not use FM jitter ("sources") to compute jitter, it is likely to give you a wrong answer, use PM, all harmonics, integration limit Fo/2 and specify the crossings.
- Oscillator circuit: if you are pretty sure that your output buffer is clean, you can use FM jitter ("sources"), full maxsideband, integration limit Fo/2; otherwise use PM as above.