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Design >> Analog Design >> PNoise analysis of PFD+Charge pump
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Message started by nandy on Jan 24^th, 2008, 4:31am

Title: PNoise analysis of PFD+Charge pump
Post by nandy on Jan 24^th, 2008, 4:31am

Hello people

I need to find the phase noise contributed by a phase/frequency detector + charge pump combination which are part of a frequency synthesizer. When the synthesizer is locked, both the inputs to the PFD are of the same frequency and hence the beat frequency is zero (albeit there will be reference feedthrough). But I cannot do a PSS/Pnoise analysis with a 0 beat frequency. How do I proceed?

I tried measuring phase noise while having a finite frequency difference too, but the simulation never seems to converge however much I change the tstab value or if I make it conservative or liberal. BTW I use cadence spectre's analog environment for the simulations.

Thanks in advance

Title: Re: PNoise analysis of PFD+Charge pump
Post by Frank Wiedmann on Jan 24^th, 2008, 5:57am

"Beat frequency" is a bit of a misnomer, "fundamental frequency" would probably have been a better choice. Simulate your circuit in the locked state and enter the lowest frequency in your circuit as "beat frequency" (or the longest period as "beat period").

Title: Re: PNoise analysis of PFD+Charge pump
Post by nandy on Jan 24^th, 2008, 7:40am

But Frank, since charge pump is shaped by the low pass filter in the loop, wont I be interested in noise near DC rather than at the reference frequency (since reference frequency is many times higher than loop bandwidth, and any noise at these frequencies will be suppressed anyway)?

Title: Re: PNoise analysis of PFD+Charge pump
Post by Frank Wiedmann on Jan 24^th, 2008, 11:12pm

You can do an absolute frequency sweep in the pnoise analysis and examine the noise at any frequency you like.

Title: Re: PNoise analysis of PFD+Charge pump
Post by nandy on Jan 24^th, 2008, 11:37pm

OK. I'll try that. Thank you Frank.