[SI-LIST] Re: Relevance of Common Mode Return Loss

  • From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
  • To: "'Randy May'" <randy.may@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 16 May 2008 12:47:21 -0500

Hi folks-

I thought Randy asked an interesting question about the common return loss
spec. I don't recall seeing any responses. 

I'd like to hear comments. I have seen a number of specs that call out a
differential impedance spec and a common impedance spec, as well as a
differential and common return and insertion loss spec.

When I asked engineers about it, all I could got was, this was the behavior
of the reference systems we built and so we expect all future systems to
meet this spec.

Other than the potential of saturation of receivers if the common signal
gets too large, and the EMI problem if the common signal gets out on
external cables, is there another compelling reason to spec the common
impedance or return or insertion loss for a cable or interconnect link?

If the differential performance is met, is there a performance reason for a
common signal spec?

Thanks for your comments.

--eric

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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Randy May
Sent: Thursday, May 08, 2008 1:27 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Relevance of Common Mode Return Loss

Greetings,
I've been trying to understand the relevance of common mode return loss in
high speed specifications.  I thought that most differential receivers would
reject common mode noise on a link, and amplify the differential, making the
differential spec far more important.  An example spec is PCI Express Gen2
which calls for <= -6db from 50MHz to 2.5GHz.  What is the impact if I
violate the common mode return loss spec?  What is the benefit to me beating
it?

I've also noticed that some spec's have a common mode return loss
requirement on the receiver and not on the transmitter.  Any thoughts on why
this might have been done?

Thanks


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