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Measurements >> Phase Noise and Jitter Measurements >> supply noise sensitivity: PSS+PAC or PSS+PXF
https://designers-guide.org/forum/YaBB.pl?num=1376500816

Message started by sacbr on Aug 14^th, 2013, 10:20am

Title: supply noise sensitivity: PSS+PAC or PSS+PXF
Post by sacbr on Aug 14^th, 2013, 10:20am

Hello,

I am trying to find the effect of supply noise on the jitter of a delay line (a set of buffers in series). When I went through various documents on this website, I found the following options

1. PSS + PAC with the sampled option.

2. PSS + PXF with sampled option

In the both the cases sampled option is selected since only the jitter at zero crossing is important. http://www.designers-guide.org/Forum/YaBB.pl?num=1224609785

Let me start with explaining one setup. When I simulate PSS + sampled PAC, I set PAC magnitude = 1 in my power supply voltage source. Set the the maximumsideband = 0, and sweep the input source from 10khz to 10Ghz. The input clock to the delay line is 1GHz.

I am a little confused with the output spectrum that I see with the above simulation. Since it is sampled PAC, I was expecting the spectrum to repeat every 1GHz (fundamental tone), like in a PSS + sampled PNOISE analalysis. However, in the PSS + sampled PAC I see that the spectrum does not repeat. Can someone explain, what is wrong with my understanding?

Also, if wanted period jitter (instead of absolute jitter) in a sampled pnoise simulation, I would multiply the output spectrum with (1-z-1) and divide by slope of the output waveform at the transition point and integrate from 0 - FCLK/2. How do I find the period jitter in a sampled PAC analysis?

All my questions basically come from the confusion that the output spectrum is not repetitive, like in a sampled system.

Thanks for help.

Title: Re: supply noise sensitivity: PSS+PAC or PSS+PXF
Post by Ken Kundert on Aug 14^th, 2013, 6:34pm

It would be perfectly repetitive if it were sampled with an ideal samplefunction. In fact, it is sampled with something that has width. Roughly the width of a time step. Thus, what you should see is a repetitive spectrum that rolls off slowly at high frequencies.

-Ken

Title: Re: supply noise sensitivity: PSS+PAC or PSS+PXF
Post by Frank Wiedmann on Aug 16^th, 2013, 4:02am

What Ken is referring to is the action of the sampling function by itself. However, you also have a transfer function in front of this sampler which in general is not periodic at all, so you should not be surprised if the resulting sampled pac or sampled pxf spectrum is not periodic as a function of the input frequency.

The reason for the sampled pnoise spectrum being periodic is that the contributions from all noise input frequencies are combined and that the result is shown as a function of the output frequency.

The result of the sampled pac or sampled pxf analysis is a transfer function. In order to get the jitter due to a supply noise input signal at a certain frequency, you simply divide the result by the slope of the output signal at the crossing point (see also http://www.designers-guide.org/Forum/YaBB.pl?num=1224609785/15).