[SI-LIST] Re: Relevance of Common Mode Return Loss
- From: "Lynne D. Green" <lgreen22@xxxxxxxxxxxxxx>
- To: "'Eric Bogatin'" <eric@xxxxxxxxxxxxxxx>, "'Randy May'" <randy.may@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 16 May 2008 11:38:03 -0700
Hi, Eric and Randy,
Regarding common mode reflections (various causes, such as component
mismatch, etc.): any imbalance converts part of the common mode energy to
differential, which in turn messes up the differential signal. Minimizing
common mode reflections minimizes effects of mismatch.
My guess is that the engineers you talked to meant "we know the reference
design works, and any changes are the user's headache". Which is not to say
changes can't be made, just that they offer no guarantees.
Simulation could be used to investigate which changes could be made safely.
However, one would want to include the potential mismatches to cover all
corners. If only the I/O corners are varied and nominal components are used
everywhere else, there is a risk that the simulation checks pass but the
system fails.
Best regards,
Lynne
"IBIS training when you need it, where you need it."
Next public IBIS seminar: Oct 9-10, Fremont, CA.
Dr. Lynne Green
Green Streak Programs
http://www.greenstreakprograms.com
425-788-0412
lgreen22@xxxxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Eric Bogatin
Sent: Friday, May 16, 2008 10:47 AM
To: 'Randy May'; si-list@xxxxxxxxxxxxx
Cc: eric@xxxxxxxxxxxxxxx
Subject: [SI-LIST] Re: Relevance of Common Mode Return Loss
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
**************************************
Dr. Eric Bogatin,
Signal Integrity Evangelist
Bogatin Enterprises, LLC
Setting the Standard for Signal Integrity Training
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San Jose, EPSI, BBDP, Sept 29-Oct 2
****************************************
-----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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