But that is not what I said! I don't think it is easy to spot this problem by looking at the symbol response. A phase delay vs. freq. plot would be better. Thanks, Vinu Chris Cheng wrote: > If you believe you have an ISI resonance pattern that will go down to 1667 > post cursor, you've got a problem bigger than just your PLL loop bandwidth. > > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Vinu Arumugham > Sent: Tuesday, September 16, 2008 12:21 PM > To: steve weir > Cc: Steve Waldstein; si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Re: Majic Fbaud/1667 for CDR bandwidth > > > > Steve, > Regarding, "generally in a CDR scheme we want to track at as high a rate > as > we can.", there is at least one situation where tracking at a high rate > can degrade performance. > When an interconnect has a resonance that causes pattern dependent prop. > delay variations, a clock like pattern can drag the sampling point away > from the middle of the eye. When the data pattern changes back to random, > one can encounter errors. With fast tracking one would need shorter > clock-like sequences to trigger this failure. With a scrambled data > stream and a CDR that only reacts to long clock-like sequences, the > probability of such errors can be reduced below the BER of interest. > > Thanks, > Vinu > > > steve weir wrote: > >> 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 >>>> >>>> >>>> >>>> >>>> >>>> ------------------------------------------------------------------ >>>> To unsubscribe from si-list: >>>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >>>> >>>> or to administer your membership from a web page, go to: >>>> //www.freelists.org/webpage/si-list >>>> >>>> For help: >>>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >>>> >>>> >>>> List technical documents are available at: >>>> http://www.si-list.net >>>> >>>> List archives are viewable at: >>>> //www.freelists.org/archives/si-list >>>> or at our remote archives: >>>> http://groups.yahoo.com/group/si-list/messages >>>> Old (prior to June 6, 2001) list archives are viewable at: >>>> http://www.qsl.net/wb6tpu >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>> >> >> > > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > List technical documents are available at: > http://www.si-list.net > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > > > This email and any attachments thereto may contain private, confidential, and > privileged material for the sole use of the intended recipient. 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