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Phase noise to phase jitter for square waves

yizh

Hi,

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!

 

Parents

  • Frank made exactly the same suggestion that I was going to make - you could use an svcvs source from analogLib to describe the filter in the s-domain. 

    If maxsideband is 1, you are only including the noise contributions from sidebands +/-1 and 0 (so you will get very high up-converted flicker noise if you sweep near to the carrier frequency). You won't get noise contributions from higher frequencies "folded" by the harmonics of the carrier because you simply are not including them. Sweeping over a wide frequency range is NOT the same as including lots of sidebands - you are sweeping the output frequency yes, but you've removed the simulator's ability to compute all the transfer functions from the noise sources to the output - you've only included three noise transfer functions.

    I think the approach you're suggesting is flawed - best is to include the filter in your circuit (it can be ideal), and then use the PMjitter approach.

    Andrew.

Reply

  • Frank made exactly the same suggestion that I was going to make - you could use an svcvs source from analogLib to describe the filter in the s-domain. 

    If maxsideband is 1, you are only including the noise contributions from sidebands +/-1 and 0 (so you will get very high up-converted flicker noise if you sweep near to the carrier frequency). You won't get noise contributions from higher frequencies "folded" by the harmonics of the carrier because you simply are not including them. Sweeping over a wide frequency range is NOT the same as including lots of sidebands - you are sweeping the output frequency yes, but you've removed the simulator's ability to compute all the transfer functions from the noise sources to the output - you've only included three noise transfer functions.

    I think the approach you're suggesting is flawed - best is to include the filter in your circuit (it can be ideal), and then use the PMjitter approach.

    Andrew.

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