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Measurements >> Phase Noise and Jitter Measurements >> About PLL in band noise question
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Message started by easyads on May 22^nd, 2008, 2:14am

Title: About PLL in band noise question
Post by easyads on May 22^nd, 2008, 2:14am

As we know, PFD, CP, loop filter , N divider,VCO all contribute in band noise. But which is the major in band noisesource?

If the on Time Ton of Charge pump is very small (about several ns), the noise of CP will be very small. And the VCO noise is high passed, so VCO contributes neglible in band noise.

In Dean's book "PLL performance and simulation", his idea is that the PFD contributes the majority of in band noise. And he thinks the if the comparison frequency increases, the noise of PFD increases

Please help me!

Title: Re: About PLL in band noise question
Post by KE5FX on Jun 10^th, 2008, 7:15pm

Reference multiplication (20*log(N)) is the dominant factor in most integer-N PLLs, as far as inband noise is concerned. Only loops designed with a very quiet reference or a low N factor will be affected by PFD/counter noise.

The higher the comparison frequency, the larger the proportion of correction pulses from the PFD will be, relative to that frequency. This accounts for the 10*log(Fcomp) term quoted by Banerjee.