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Jitter relation between VCO & loop divider output (Read 874 times)

keikei

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Jitter relation between VCO & loop divider output
May 28^th, 2008, 7:01am

In a PLL locked to a reference frequency, what is the relation between the jitter of VCO output ('Jvco') and the jitter of the signal fedback to PFD input after the loop divider ('Jdiv') ? assuming the div ratio is N.
In a certatin paper the author gives Jvco = N * Jdiv. But according to some early discussion here, most people agree that Jvco = Jdiv. So I am totally confused.
Is the effect of divider on jitter dependent on its location, i.e., in loop or out of loop?

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Stefan Senior Member Offline Posts: 124	Re: Jitter relation between VCO & loop divider out Reply #1 - May 28^th, 2008, 7:08am It's just some basic stochastic equation the standard deviation of N statistical indepedent events, each having a standard deviation of J1 is JN=sqrt(N)*J1. So neither one of the formulas you spoke about is correct. And please don't talk about "certain papers", quoting them is more efficient here
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keikei New Member Offline Posts: 7 beijing	Re: Jitter relation between VCO & loop divider out Reply #2 - May 28^th, 2008, 8:39am yeah, I should have specified the paper name: Time-to-voltage converter for on-chip jitter measurement, IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 52, NO. 6, DECEMBER 2003 by Tian Xia and Jien-Chung Lo
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Visjnoe

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Re: Jitter relation between VCO & loop divider out
Reply #3 - Jun 6^th, 2008, 11:19am

Dear,

Jvco = Jdiv (for jitter components within the PLL bandwidth).

The formula with the sqrt(N) relationship does not hold. It only applies for calculating the jitter of a divider chain constituted out of N unit elements (e.g. div 2). When the jitter of a unit element Junit is known, the total jitter of the chain (divider) is given by sqrt(N)xJunit.

The phase noise before and after the divider has a 1/N^2 relationship. Since the period becomes N times longer after the division,
the jitter is the same.

Regards

Peter

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