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Hi Richard,
I thank you for the reply.
This really helps me understand your reasoning behind using SCD21 in the SCMR_CH calculation ― and the value of taking it one step at a time when dealing with mode conversion.
Regards,
Hansel D'Silva
From: Richard Mellitz <Richard.Mellitz@xxxxxxxxxx>
Sent: Tuesday, June 24, 2025 2:45:14 AM To: Hansel DSilva <Hansel.D'Silva@xxxxxxxxxxxxxxxx>; STDS-802-3-B400G@xxxxxxxxxxxxxxxxx <STDS-802-3-B400G@xxxxxxxxxxxxxxxxx> Subject: RE: [802.3_B400G] FYI: Proposal to specify a control for skew in IEEE P802.3dj 2.0 CAUTION: EXTERNAL EMAILI realize there's a fundamental lack of DC-to-CD duality, and things get complex quickly. The core idea is to quantify or constrain what’s missing in COM that leads to interoperability failures. COM evaluates differential-mode (DM) performance but can overlook the impact of common-mode (CM) impairments, especially those introduced at the system periphery. Take skew as a simple concrete example. If the periphery introduces no skew, then COM typically evaluates the channel correctly. But if skew is present ― say, from a transmitter ― it produces common-mode voltage. That CM voltage, affected by CM return loss (CM RL), can convert into differential-mode energy and reach the receiver. This effect is not captured in COM, which assumes ideal DM signaling. In this way, CM-to-DM conversion is a key contributor to channel interoperability. The reverse ― DM to CM ― is primarily a concern for emissions or receiver sensitivity. Returning to the skew example: I considered incorporating equalization into the analysis. However, that would require rethinking the CM RL spec. Specifically, we'd need to shift from a static CM RL mask to a CM RL power-based SNR metric, analogous to how SNR_MDFEXT (proposed) models crosstalk noise. This would allow defining an SNR_CMRL. SNR_CMRL is easily converted into modal gammas which are used to create the channel's CM transfer function, yielding a modal pulse response for which equalization could be applied meaningfully. But ― one step at a time. … Rich
From: Hansel DSilva <00004708d0869f3e-dmarc-request@xxxxxxxxxxxxxxxxx>
Hi Richard,
In the attached proposal regarding using SCMR_CH (for the channel) to control skew and other imbalances, I noticed that only SCD21 is mentioned. Would the proposal include computing SCMR_CH using SDC21 as well?
Below is my reasoning behind this question.
- Equation 178-1 in D2P0. - D2p0 mentions VCM_FB as the full-band peak-to-peak AC common-mode voltage defined by the method specified in 179.9.4.1 and measured with the transmitter equalization set to “no Equalization”. - I interpret this as calling out the differential-mode to common-mode insertion loss (SCD21).
- Equation 178-6 and 178-7 in D2p0. - It calls out differential-mode to common insertion loss (SCD21) and common-mode to differential-mode insertion loss (SDC21).
- Assuming the throughs are (1)---(2) and (3)---(4). -Differential insertion loss is reciprocal, meaning SDD21 ≈ SDD12. a. SDD21= 0.5*(S21 - S23+ S43- S41). SDD12= 0.5*(S12 - S32+ S34- S14). 4. - Mode conversion terms are not strictly reciprocal: a. SCD21 ≈ SDC12 and SCD12 ≈ SDC21. 5. - However, SCD21 ≠ SCD12, as shown in the following equations: a. SCD21= 0.5*(S21 - S23 + S41 - S43). a. SCD12 = 0.5*(S21 + S32 - S14 - S34). a. SDC21 = 0.5*(S12 + S23 -S41 -S43). a. SDC12 = 0.5*(S12 - S32 + S14 - S34).
Regards, Hansel D'Silva
From: Richard Mellitz <richard.mellitz@xxxxxxxxx>
CAUTION: EXTERNAL EMAILHi All, The has been presented some this before. This this attached is a proposal for using SCMR_CH (for the channel) to control skew and other imbalances. The first half, SCMR and V_CM was controlled in the transmitters. This proposal it to complete the process for the channels. Comments? Thoughts? Regards, Richard Mellitz Samtec
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