[SI-LIST] Re: effect of ref clock's phase and Frequnecy on SERDES operation
- From: Mike Brown <bmgman@xxxxxxxxxx>
- To: prasad <hariprasad.palli@xxxxxxxxx>
- Date: Sun, 11 Sep 2011 13:43:41 -0500
On 9/8/2011 3:26 PM, prasad wrote:
> Hi everyone,
> i need your help in understanding effect of ref clock on serdes operation.
>
> I am trying to understand one design, where IBM's 6.4Gbps serdes was used.
> I went through its architecture, and i could see that the same ref clock is
> being used for Rx and Tx.
>
> i would like to understand the effect of frequency variation and Phase
> variation of the reference clock provided on both Tx side and Rx side.
>
> I assume on RX side, there might be dependancy on the frequnecy but not
> phase, where as on Tx side, there would be dependancy on both phase and
> frequency. Please correct if i have mistaken.
>
> It would help , even if you point to good source of document related to this
> SERDES and CDRs.
>
>
> Thank you everyone in advance....
>
>
> Regards,
> prasad
>
As an "a while back" (0.8 u, IIRC) IBM SerDes user, I doped out the
design concept of their clock recovery circuit. I don't believe I'm
violating any NDA that was in force at the time. That older design used
the same ref clk for both transmit and receive blocks. However, they
cleverly built a block called a "PHASE ROTATOR" into the clock
recovery of the RX block. It generated a multi-phase clock at the data
frequency (or F/2?) and used a mux to apply one of those phases to the
data recovery register. When a data edge appeared early or late in the
register, the phase selection was changed by an amount which would tend
to center the next edge when it appeared.
In essence, they generated a variable frequency/phase data clock,
coarsely locked to the data transitions, from a fixed frequency/multi
phase reference. At the time, I was pretty impressed by the technique.
Now that I'm retired, I'm still impressed, but much less concerned!
Since the supplied reference controls the TX clocking, its phase
stability and frequency tolerance will directly effect the BER of the link.
All that to say that using a single fixed reference freq/phase for both
TX and RX has been done for a while now; I'm not sure how they are doing
it in today's technology.
Mike
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