[SI-LIST] Re: to what extent recovered clock depends on the serdes ref clock

  • From: "T.K. Jeon" <tkjeon@xxxxxxxxxxx>
  • To: "hariprasad.palli@xxxxxxxxx" <hariprasad.palli@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 6 Apr 2012 14:57:08 -0700

CDR tracks only in-band modulation like SSC and low frequency sinusoidal 
modulation.

If the '+100ppm' is directly added to the frequency without modulation, then 
you are running the serdes with a different refclk, which is 161.146MHz, 
instead of 161.13MHz. Many serdes applications calls for using this kind of 
timing scheme called plesiochronous timing where Tx runs at a slightly 
different clock frequency from Rx or vice versa.

Regards,
TK

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of prasad
Sent: Thursday, April 05, 2012 9:55 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] to what extent recovered clock depends on the serdes ref 
clock

Hi everyone,
might this be a fundamental question. If this can be answered by some self
reading, please point me to the source of such notes. Thanks in advance.

My question is , to what extent the recovered clock of the CDR inside the
SERDES depend on the reference clock given to it?

I believe the RX side of the SERDES uses the input ref clock given to it as
a guiding clock , to track the incomind data signal.

which means, for eg, if

I/p serial signal is at 10.3125Gbps ( 10GELAN)
serdes is of BW 64 on parallel side  ( FYI this is IBM HSS11G serdes)
ref clock given is  161.13MHz  (  10.3125G/64, this gets multiplied by
HSPLL to generate the guiding clock fr the serdes)

in this case, i should be/am getting, the recovered clock/64   as 161.13MHz.

But what happens if my ref clock is 161.13MHz+100ppm?


shouldnt the recovered clock/64 be still 161.13MHz, as the CDR shuld be
still tracking 10.3125GHz? From an experiment I did, i observed the
recovered clock/64 be around 161.13MHz+100ppm (roughly).



I am not sure why this happens.  It would be great if any of you kindly
help me understand this.

Thanks one and all...


Best Regards,
VEnkat


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