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Design >> High-Speed I/O Design >> Alexander PFD
https://designers-guide.org/forum/YaBB.pl?num=1274900372

Message started by neoflash on May 26^th, 2010, 11:59am

Title: Alexander PFD
Post by neoflash on May 26^th, 2010, 11:59am

Hi,

Almost all BB-PD used in CDR have no frequency detection capability.

What kind of behavior they will show if there is large frequency offset between data and local sampling clock?

Thanks,
Neo

Title: Re: Alexander PFD
Post by love_analog on May 31^st, 2010, 7:56am

They will a parallel Frequency detector loop. Once the VCO is brought within range, you will switch over to the PD loop

Title: Re: Alexander PFD
Post by casual on Jun 2^nd, 2010, 9:49pm

typically BB-PD has a narrow LPF bandwidth compared to linear PD

Title: Re: Alexander PFD
Post by casual on Jun 11^th, 2010, 5:37am

Only feedforward CDR architecture (aka oversampling type) does not have frequency detector.

Title: Re: Alexander PFD
Post by KangSub on Jul 24^th, 2010, 1:32am

hi,

followings are my opinion.

If there are large frequency offset between clock and data, BBPD will produce the random sequence of UP and DN outputs.

BBPD uses at least 3-sample in consecutive 2-UI for phase detection between clock and data and produces valid UP and DN according to the position of data transition.
on the other words, BBPD detects the position of data transition relative to that of clock, using consecutive 3-samples in 2-UI.
so, the frequency of clock and data have to be equal.

if there exist large frequency offset between data and clock, then, BBPD cannot sample 3-sample in consecutive 2-UI and cannot operate properly.
in that case, the 3-samples have no information about the position of the data transition.
therefore, the output of the BBPD is not meaningful.
if the input data is PRBS, because the probability that sampled bit is 0 or 1 is always same, the UP and DN output of BBPD will also random sequence.

I don't think that Alexander BBPD can detect any frequency information.

thanks,

- KangSub

Title: Re: Alexander PFD
Post by loose-electron on Aug 31^st, 2010, 4:39pm

Um... this is covered in a dozen textbooks...

look into pull in range, and frequency acquisition vs phase acquisition.

Title: Re: Alexander PFD
Post by neoflash on Aug 31^st, 2010, 5:31pm

Thanks for bring this reference source up.

Would you like to list 1-2 text books which you think are best in describing this issue?

Thanks.

loose-electron wrote on Aug 31^st, 2010, 4:39pm:

Um... this is covered in a dozen textbooks...

look into pull in range, and frequency acquisition vs phase acquisition.