[SI-LIST] Re: Majic Fbaud/1667 for CDR bandwidth
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>
- Date: Tue, 16 Sep 2008 16:13:28 -0700
Steve,
I am referring to "phase jumps" created by XAUI CJPAT like patterns on
an interconnect with resonance. Standards like CEI recommend
coding/scrambling schemes to reduce the probability of such patterns.
Imagine a horizontal line through the middle of the eye.
C----R-c---r----C
R-C is the eye opening with random patterns.
C-C is the eye opening for a clock pattern (low ISI, wide opening but
offset from R-C because of resonance)
c is the optimal sampling position for the clock pattern
r is the optimal sampling position for the random pattern
Tracking speed affects the number of bit times needed to move from r to
c. A "lone pulse" after a clock like pattern can cause a failure because
the CDR does not have enough transitions/time to get back from c to r.
For an interconnect without resonance:
C---R---r---R---C
R-R is the eye opening with random patterns.
r is the optimal sampling position for all patterns.
Thanks,
Vinu
steve weir wrote:
> Vinu that premise sounds questionable to me. I think any resonance
> that causes substantial movement in the timing estimate must by
> definition move the associated data. As long as the PLL is properly
> damped ( many are not ) the wide bandwidth loop should track much
> better and exhibit a lower error rate than a norow loop.
>
> Steve
> Vinu Arumugham wrote:
>> 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
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
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>>>>
>>>
>>>
>>>
>>
>
>
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