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PFD & Frequency Detector

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Full Member level 4
May 11, 2005
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what is the difference b/w PFD and a frequency detector ?
is xor a frequency detector ?
and how exactly a frequency detector detects frequency?

XOR is a phase detector.

Hi, if you know the state machine of the PFD, you can understand the PFD more clearly. You can find the state machine in many PLL books and papers.

I know PFD's working & state machine. But i want to know theoretically how a Frequency Detector works. and want an example of only Frequency Detector !

PFD's are normally used in PLL's for comparison's...PFD's are used to provide a higher acquistion range because using just a PD puts a constraint on acquisition based on the loop filter frequency. A frequency detector can be use din conjunction with the PD in the same loop. The PLL first brings the output and reference frequency together using the FD and then tries to lock the phase. The FD generates a voltage component that is proportional to the frequency difference which drives the VCO....hope this helps....

check chapter 8,,,in razavi RF book about frequency synthesizers,,,also ,,there is a good book about the PLL in general that contains what you want also and very famous done by "best",,,,,,,
in brief,,PFD: when the step in frequency changes between the two compared inputs is large so that it is out of linear region ,,an constant output voltage appears which converts to ramp after passing through the loop filter and VCO "note that VCO may considered as an integrator" so, the frequency of the VCO changes greatly in the direction of the other frequency,,,till it enters the linear region( note that the linear region determined by the max. frequency step that makes the error in phase equal 2*pi),,,so,,now the output voltage of the PFD-CP will be pulsed(not constant) and its average will drive the VCO according to the loop dynamics to makes the two frequencies in phase and equal,,,,so you can consider there is two curves one for linear which detects the phase and other for non-linear which detects the frequency,,,,,,xor is just a phase detector that fails when the two inputs have significantly differences in frequencies,,,hope that help you ....

Check the lock detection paper on this link,it may help

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