[SI-LIST] Re: Majic Fbaud/1667 for CDR bandwidth
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Steve Waldstein <swldstn@xxxxxxxxxxxx>
- Date: Tue, 16 Sep 2008 00:16:22 -0700
Steve, generally in a CDR scheme we want to track at as high a rate as
we can. We can dump the results into an elastic store and then use a
second PLL with a lower rate to smooth out the bumps for reading out the
elastic store and/or forwarding.
I don't know why XAUI has such a tall ratio. Either there is some break
in the 8b/10b pattern possible, or it seems to be about 50 times taller
than it needs to.
Steve.
Steve Waldstein wrote:
> Steve,
>
> Thanks for your answer but I'm still a little perplexed. In a PLL the loop
> bandwidth typically wants to be about a factor of 10 lower than the
> transition density in the reference clock to the PDF. But pushing the
> bandwidth lower will allow a noiser (more jitter) reference clock at the
> expense of seeing increased VCO jitter. The opposite it true where you use a
> higher loop bandwidth to clean up the VCO but you suffer from clock noise
> passing through the loop bandwidth that causes output jitter.
>
> I'm sure there is a similar analogy for the CDR. A lower loop bandwidth
> should produce a cleaner recovered clock but makes the loop less agile to
> data changes. A higher loop bandwidth makes the loop more agile but produces
> more jitter on the output.
>
> Lets use an example for discussion. XAUI has Fbaud = 3.125 Gb/s and 8b/10b
> (or 10Q) encoded. Yet its corner frequency is set at 3.215/1667 = 1.87 MHz.
> Is this because XAUI want to recover a clock and recreate it to some kind of
> PPM accuracy similar its input spec of +/- 100 PPM? I know SONET had
> repeaters in it where the clock recreation was important but on most serial
> links that's not the case. So since you said Fbaud/30 was typically
> sufficient to recover the day why burden the receiver with such a narrow
> loop bandwidth?
>
> Is it really related to the fact that at +/- 100 PPM one skip is inserted
> every 5000 symbols so the 1667 provides margin to this by a factor of 3?
>
> I've also seen calculation that predict the jitter of a sinusoidal
> modulation of the carrier that relate to the equivalent PPM. It the corner
> really set to handle this type of issue? And not ability to recover the
> data?
>
> I know these are a lot of questions but your answer doesn't help understand
> why these standards have chosen such a low loop bandwitch.
>
> Steve W.
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of steve weir
> Sent: Sunday, September 14, 2008 10:38 PM
> To: Steve Waldstein
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Majic Fbaud/1667 for CDR bandwidth
>
> 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
>>
>> __________________________________
>>
>> Steve Waldstein
>>
>> E-mail: swldstn@xxxxxxxxx
>>
>> Mobile: (207) 749-6260
>>
>> Home: (207) 885-0594
>>
>>
>>
>>
>>
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>
>
>
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