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May 24th, 2019, 9:08am
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Is "phase jitter" short term jitter or long term accumulated? (Read 126 times)
Homer
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Is "phase jitter" short term jitter or long term accumulated?
Mar 28th, 2019, 9:01am
 
Hi,

Now I'm confused about the "phase jitter" after studied on the internet.

All the materials say the phase jitter is the jitter integrated from the phase noise. There is no problem for this defintion.

Some papers/books think it's period jitter which is short term. In candence pm jitter plot, you can chose how many cycles. I assume 1 cycle means the short term period jitter and N cycle, when N is quite big, is the accumulated long term jitter.

But some papers say phase jitter is TIE jitter. And TIE jitter is definetly accumulated jitter for long term.

So which one is correct?
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Re: Is "phase jitter" short term jitter or long term accumulated?
Reply #1 - Apr 15th, 2019, 3:40am
 
I don't think I have heard the term called phase jitter. Jitter is a representation of the inconsistency in timing of the noisy output clock with respect to an ideal clock which should have been absolutely based on the period dictated by the input or the autonomous system.

Now this randomness can be represented as samples whose magnitudes at each of the timing edges represents the difference between the noisy edge and the ideal edge. This being a waveform in time has a frequency representation. The random nature of this waveform only lets us represent it in power spectral density format. This frequency domain representation is called phase noise.

Now every random noise has 2 representations which put together define it. First is the shape of PSD and second is the shape of magnitude density function(MDF). PSD for white noise is flat, flicker is 10dB/decade. PSD for an ADC quantization noise is assumed to be white.
While MDF for uncalibrated mismatch, phase noise, voltage noise etc. is gaussian, the quantization noise is assumed to be uniform.

You can find variance from intagrating PSD or from MDF directly using probability theory. In fact PSD only tells how the same noise power is distributed in frequency. Noise power can be seen as variance of voltage/phase etc. which is why the term noise power.

Now based on long term or short term, you can apply a sinc filter represented by (1-z^(-t/T)). For t=T, we get a simple high pass filter. This means you look at one period jitter when you integrate by applying this filter on the phase noise plot. You can also think that the duration for which we see actually limits the effect of lower frequency noise to even appear effectively becoming a high pass filter. By increasing the time duration for which you're looking at the accumulated noise, your HPF cut off goes towards DC.
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