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Re: [802.3_COM] RLM

Problem I have with RLM is that there are several combinations that lead to the same RLM number.

e.g. I think [-1 -0.35  0.33333 1] (asymmetric signal set) will yield the same RLM number.

Likewise [-0.98 -0.333 0.333 1] and variations.

 

Granted, moving two signal points is probably a “fair” representation.

 

From: Richard Mellitz <000014533bad0b9c-dmarc-request@xxxxxxxxxxxxxxxxx>
Sent: Wednesday, March 26, 2025 5:52 PM
To: STDS-802-3-COM@xxxxxxxxxxxxxxxxx
Subject: [802.3_COM] RLM

 

Caution: This email originated outside of Semtech.

 

Hi all,

 

I was looking at adjusting the symbol alphabet used to compute COM ISI, crosstalk, and jitter for R_LM. Presently we just use a mean shift in the eye levels by scaling As by R_LM.

 

The symbol alphabet used in COM PAM4 is presently [ -1 -1/3 1/3 1] or 2*(0:L-1)/(L-1)-1.

I think the symbol alphabet for pam4 would be [-1 -.35 .35 1] for R_LM= 0.95 or in general scale level 1 and L-1 by 2-R_LM for PAML.


The last time I tried this the impact on COM was small (< 0.1 dB). It could be more important for higher PAM.

 

This has been suggested to mebefore.

Thoughts?

 

… Rich

 

 

Richard Mellitz, Signal Integrity (SI) Engineer

Samtec Southeast

Office: 803-908-4411

www.samtec.com


 

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