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Design >> Mixed-Signal Design >> about PFD, plz
https://designers-guide.org/forum/YaBB.pl?num=1245228641

Message started by taotaohai on Jun 17^th, 2009, 1:50am

Title: about PFD, plz
Post by taotaohai on Jun 17^th, 2009, 1:50am

I design a fractional-N pll,boss said that the lock time should be control in 20us,but see from the simulation,the result is not fine.the plot shown have some problems,but my topic focus on the PFD circuit.
My PFD is the conventional 2 dff+feedback,I think that this type PFD only can discriminate phase in a word ,many book describe it can discriminate frequency,but its principle is still begin from phase.
Now my wish voltage is 1v,the simulation show that the ref and feedback signal is almost reversed(means these two signal' frequency are same but phase diff pi@that time)near 1V,but the voltage is still up!because the PFD see it as phase difference,so I wonder is there any way could help me realize frequency discriminate at my 1V? If done,the system could lock fast........
Thank you

Title: Re: about PFD, plz
Post by raja.cedt on Jun 17^th, 2009, 3:06am

hi,
the pfd what you have shown will discriminate frequency but it operates based on phase detection (basically it counts no of leading edges passing through, so if ref signal frequency is more means more no of edges will come and up will be high)...In your waveforms whats that under damped waveform? i guess it is control node voltage..

thanks,
rajasekhar.

Title: Re: about PFD, plz
Post by taotaohai on Jun 17^th, 2009, 4:13am

[quote author=raja.cedt link=1245228641/0#1 date=1245233216]hi,
In your waveforms whats that under damped waveform? i guess it is control node voltage..

Hi,3Q first!
Do you mean my damp is very small,so the wave is oscillating? YES,maybe my damp is not so large enough,but if my PFD could detect the frequency immediately,then the wave should be small ,and the lock time alse shorts, is that right?