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Measurements >> Phase Noise and Jitter Measurements >> jitter due to filter leak
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Message started by Pravinkumar on Apr 2^nd, 2007, 8:43pm

Title: jitter due to filter leak
Post by Pravinkumar on Apr 2^nd, 2007, 8:43pm

Hi,
In specs of PLL, am trying the calculate the contribution of the filter leakage current to the overall jitter.
With filter leak current in hand will this formulae,
leakage current * 10 * (1/minimum reference clock frequency)
yield me the jitter value due to this leakage current?
If somebody has come across this sort of calc before, putforth your suggestions please.

Title: Re: jitter due to filter leak
Post by mg777 on Apr 3^rd, 2007, 4:21am

By dimensional analysis you're missing a current in the denominator, and the only current in the problem is the charge pump source. I can't see any reason for the 10.

Do PLL designs these days still use an external cap?

M.G.Rajan

Title: Re: jitter due to filter leak
Post by Pravinkumar on Apr 4^th, 2007, 3:58am

Thanks for replying rajan.

Yes, i know there is a big flaw in terms of the dimensions. But this emperical formula is what is suggested. May be i guess i have to divide this term by the capacitor value through that path.
And I dint get ur question on PLL design!!

Title: Re: jitter due to filter leak
Post by joel on Jun 6^th, 2007, 4:09pm

Every PLL I've designed or worked on this millenium has had an internal cap. Usually (always, I think) gate-cap of a native NFET, from 50pF to 2nF (that one was called the 'solar panel').

Actuall leakage is typically very small. I've watched the frequency remain roughly constant for seconds after the PFD/qpump is turned off.

If leakage is present, the loop has to correct for it. So the amount of leakage-induced jitter will be a function of the loop bandwidth, the filter-cap size, the VCO gain. Leakage will lower the control voltage by whatever charge it drains off the filter-cap between up/dwn pulses. Bigger cap, less delta-V. Lower K0, less delta-F. Higher bandwidth, less time for leakage to discharge the filter cap, less delta-V.