Re: [802.3_B400G] Proposal for Impedance change for IEEE P802.3dj d2.0

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Re: [802.3_B400G] Proposal for Impedance change for IEEE P802.3dj d2.0

To: STDS-802-3-B400G@xxxxxxxxxxxxxxxxx

Subject: Re: [802.3_B400G] Proposal for Impedance change for IEEE P802.3dj d2.0

From: Bill Kirkland <00001cd3c07be0a7-dmarc-request@xxxxxxxxxxxxxxxxx>

Date: Fri, 13 Jun 2025 13:58:43 +0000

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Reply-to: Bill Kirkland <wkirkland@xxxxxxxxxxx>

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Thread-topic: Proposal for Impedance change for IEEE P802.3dj d2.0

Rich may I suggest a small change to the order of operations in COM for s-parameters.

I would suggest changing the order of s-parameter manipulations (if they are still the same as a couple of years ago).

I believe you currently read in the channel s4p file and immediately convert to Sdd and then add in package and die s-parameters.

This works if both Rdies are 50 ohms. However if they are not, then to get COM to agree with ADS (which is really realy nice to have)
then you should:

Read in S4p’s, Convert all addition S-parameters to S4p’s, Cascade S-parameters, then re-normalize the s-parameters from 50 ohms to Rdie (both ends have to be the same), then finally convert to Sdd before computing the frequency domain transfer function.

The error in COM is not particularly large but is noticeable under certain conditions when trying to compare COM SBR with those from ADS when not using Rd = 50.

Below slide is taken from a presentation I was working on in 2020.

From: Richard Mellitz <000014533bad0b9c-dmarc-request@xxxxxxxxxxxxxxxxx>
Sent: Thursday, June 12, 2025 4:49 PM
To: STDS-802-3-B400G@xxxxxxxxxxxxxxxxx
Subject: [802.3_B400G] Proposal for Impedance change for IEEE P802.3dj d2.0

Caution: This email originated outside of Semtech.

Hi All,

Here is a proposal for changing the reference impedance for d2.0 to 92.5 ohms differential and 23.125 ohms common mode since the target impedance for test fixture was change.

The is no change to the COM computation algorithm required. There are several sections that are touched by this.

Comments? Thoughts?

Regards,

Richard Mellitz

Samtec

Richard Mellitz, Signal Integrity (SI) Engineer

Samtec Southeast

Office: 803-908-4411

www.samtec.com

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