[SI-LIST] Re: Majic Fbaud/1667 for CDR bandwidth
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Steve Waldstein <swldstn@xxxxxxxxxxxx>
- Date: Sun, 14 Sep 2008 19:37:43 -0700
Steve the loop B/W has to do with:
The available repetitive data rate.
Reasonable phase / gain margin for the loop filter.
Each of the various data transmission standards are different in the way
that they can mess up a CDR, with the net result that many standards
need very tall ratios between Fbaud and Fcorner. Basically, you can
easily achieve very stable operation by setting Fcorner = Frepeat / 5.
With some care you can set it to Frepeat / 3, where Frepeat is the
guaranteed lowest repetitive full 1-0 cycle. For a pure 8B/10Q coded
link, Fcorner can be as high as Fbaud / 30 and work well.
As Chris Cheng has bemoaned, TIE and jitter in general both get worse
with taller ratios as the VCO drifts ( or is disturbed by things like
PDN noise ) over more bit intervals without the benefit of corrective
feedback.
Steve.
Steve Waldstein wrote:
> I know many serial specifications place the corner frequency of a CDR at
> Fbaud/1667. I also know that the FC-MJSQ discusses how this was shifted from
> the Fbaud/2500 established for SONET. What I can't find is a good discussion
> on how to set CDR loop bandwidth for new serial specification. It appears
> there's some relation the desired frequency accuracy or ppm but haven't
> found a good derivation. Can anyone provide a good reference relating to
> choosing loop bandwidth based on desired output jitter or what ever else
> helps set this corner frequency.
>
>
> Thanks.
>
>
>
> Steve
>
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> Steve Waldstein
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