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PLL's an more important Low pass folter - RF Cafe Forums

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Post subject: PLL's an more important Low pass folter Posted: Thu Aug 09, 2007 2:27 am


Joined: Wed Sep 15, 2004 3:15 am

Posts: 4

Location: Huntington beach

Well first i would like to know if anyone knows any online places that have goos info on pll's? searching i only came up woth some very basic stuff like wkipedia.

But my main question is on the after the phase comparator the lpf (can this be a band pass?) and how exactly does it effect settleing time. I would think a narrower bandiwidth will lock loop faster since it would pass less noise. And i thought that the filters job was just to pass dc because thats what input to the varactor on the vco?

Sorry for the "newb" question but i actually never seen a pll circuit, we use it internal to the chip so i cant probe around.



Post subject: Posted: Thu Aug 09, 2007 5:39 am


Joined: Fri Feb 17, 2006 12:07 pm

Posts: 218

Location: London UK

Try www.circuitsage.com

That site provides design guides and design routines for Mathcad

Hope it helps.



Post subject: Posted: Thu Aug 09, 2007 10:36 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

The only filter possible in PLL is a LPF. The purpose of the LPF is to smooth the output of the FPD (Frequency Phase Detector) and it sets the speed of the loop.

A wide filter will cause the loop to react faster, since the filter's pole(s) is/are located far, therefore, the filter has a wider reponse. The tradeoff, as you mentioned, will be a degradation in phase noise and vice versa.



Post subject: Posted: Thu Aug 16, 2007 4:33 pm


Joined: Wed Sep 15, 2004 3:15 am

Posts: 4

Location: Huntington beach

Thanks. i had not had the chance to thank you guys been doing alot of reading.

I do understand pll better but not enough yet.

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