Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

Re: [802.3_100GEL] Resolving Even-odd jitter comments

Hi Geoff

 

John’s presentation calvin_3ck_01_1020 included a detailed explanation (slide 7) and there was some more discussion provided by Rob Sleight in the ad hoc presentation calvin_3ck_adhoc_01_091620.

 

My take is that the length of PRBS13Q is large enough that jitter within the CRU bandwidth can cause the transitions in two subsequent repetitions (separated by 8191 UI, which is 154 ns è half-wavelength of 3.2 MHz) to be at different times. This looks exactly like EOJ and can either add or subtract to the real EOJ, hence enabling bimodality in results, or just increased measured EOJ.

 

With PRBS9Q, the separation between repetitions is only ~4.5 ns, corresponding to half-wavelength of 110 MHz – jitter at such frequencies will not be tracked by the CRU, so there will be no fake EOJ.

 

Other short patterns would work too.

 

 

</Adee>

 

From: Geoff Zhang <geoffz@xxxxxxxxxx>
Sent: Tuesday, October 27, 2020 01:38
To: Ran, Adee <adee.ran@xxxxxxxxx>; STDS-802-3-100GEL@xxxxxxxxxxxxxxxxx
Subject: RE: Resolving Even-odd jitter comments

 

Hi Adee,

 

Theoretically, how do you justify PRBS9Q does not have repeatability issues?

 

Thanks,

Geoff

 

From: Ran, Adee <adee.ran@xxxxxxxxx>
Sent: Sunday, October 25, 2020 2:26 AM
To: STDS-802-3-100GEL@xxxxxxxxxxxxxxxxx
Subject: [802.3_100GEL] Resolving Even-odd jitter comments

 

CAUTION: This message has originated from an External Source. Please use proper judgment and caution when opening attachments, clicking links, or responding to this email.

 

Hi all

 

I’m sending this over the reflector to include people who have not been part of the discussion so far but may be interested.

 

We should be resolving EOJ related comments (48, 186, 189, 52, 187, 188, 127, 190) hopefully this week. We have seen presentations calvin_3ck_01_1020 and ran_3ck_01_1020 and had a discussion and some straw polls.

 

The straw polls results indicate support to the following ideas

  1. Increasing the EOJ limit to 0.025 UI
  2. Specifying or allowing a lower CRU bandwidth
  3. Use the average of the 12 transition measurements rather than the worst case

 

The suggestion to use PRBS9Q or another pattern suitable for EOJ measurement (which would be the best technique to overcome the interaction between CRU bandwidth, pattern length, and other sources of jitter) had low support in the straw polls.

 

The combination of the 3 items above does not seem to provide a good enough solution:

  • Taking the average of 12 measurements instead of worst one may be a good idea by itself, but it does not solve the main issue – the interaction above can affect all measurements in a similar way and results can still have the same repeatability issues even with this change.
  • Using lower CRU bandwidth may work in some cases, but may close the eye (due to untracked jitter) and prevent accurate measurement in other cases.
  • Increasing the limit to 0.025 UI may not be enough if the variation between measurements is large, as shown in some cases.

 

I think using a shorter pattern is the key component required for solving this issue. Results in calvin_3ck_01_1020 (backup slides) show that measurements with “PRBS9Q” (although not currently defined in the standard) are more consistent than with any other method. PRBS13Q does not provide this consistency (for reasons that were explained) and there is nothing special about this pattern that requires us to keep using it.

 

Allowing shorter patterns (with adequate criteria) without specifying a single pattern, as in the text below, would be a relatively easy change and a good solution. It may be seem that allowing more freedom in measurements opens the door to inconsistency in results, but all measurements have a mathematical basis that should work regardless of the pattern – if the specific pattern affects the measurement it is usually a bad thing (as in this case). We may choose to add a specific pattern in the future if there is a compelling reason.

 

I would suggest the following as a response to the comments above:

 

  1. Change text in 162.9.3.3 as follows

162.9.3.3 Output jitter

Output jitter is characterized by three parameters, J3u, JRMS, and even-odd jitter. These parameters are calculated from measurements with a single transmit equalizer setting to compensate for the loss of the transmitter package and host channel. The equalizer setting is chosen to minimize any or all of the jitter parameters.

J3u and JRMS are calculated using the measurement method specified in 120D.3.1.8.1. J3u is defined as the time interval that includes all but 10–3 of fJ(t), from the 0.05th to the 99.95th percentile of fJ(t).

Even-odd jitter is calculated using the measurement method specified in 120D.3.1.8.2 with the following exceptions:

  • The PRBS13Q pattern may be substituted with a shorter odd-length pattern that includes the 12 possible transitions between two different PAM4 symbols if it improves the measurement precision.
  • The corner frequency of the clock recovery unit (CRU) may be set lower than 4 MHz if it improves the measurement precision.
  • Even-odd jitter EOJ is the average of the 12 measurements defined in 120D.3.1.8.2 instead of the maximum.

 

 

  1. In 120F.3.1.3, change the cross-reference for EOJ measurement from 120D.3.1.8.2 to 162.9.3.3 to incorporate the exceptions above.
  2. In Table 162–10, Table 163–5, and Table 120F–1, change the value for “Even-odd jitter, pk-pk” from 0.019 to 0.025.

 

Discussion prior to the teleconference would be welcome.

 

Best regards

 

Adee

To unsubscribe from the STDS-802-3-100GEL list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=STDS-802-3-100GEL&A=1

To unsubscribe from the STDS-802-3-100GEL list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=STDS-802-3-100GEL&A=1