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

  • From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
  • To: "'steve weir'" <weirsi@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 16 May 2008 14:29:45 -0500

Steve-

I can understand how excessive common signal can contribute to increased
jitter of the received differential signal in the receiver due to, as you
say the finite CMRR. 

So, shouldn't the spec be on the amount of common signal that is present,
rather than on the common insertion or return loss of the interconnect?

If the worry is on the amount of common signal present, then shouldn't the
spec be about the SCD21 value, which would indicate how much common signal
would be converted from the differential signal?

Other comments?

--eric

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-----Original Message-----
From: steve weir [mailto:weirsi@xxxxxxxxxx] 
Sent: Friday, May 16, 2008 1:39 PM
To: Eric Bogatin
Subject: Re: [SI-LIST] Re: Relevance of Common Mode Return Loss

Eric, CMRR is a finite number in any receive amplifier.  Larger common 
signal components that are still within the CMR adversely impact jitter 
compared to smaller common signal components.

Best Regards,


Steve.
Eric Bogatin wrote:
> 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
> 26235 w 110th terr
> Olathe, KS 66061
> v: 913-393-1305
> f: 913-393-0929
> c:913-424-4333
> e:eric@xxxxxxxxxxxxxxx
> www.BeTheSignal.com 
>
> Upcoming Signal Integrity Classes
> Winnersh, UK, TSI, May 20, 2008
> San Diego: EPSI, BBDP, July 28-31, 2008
> San Jose, SICT, Aug 12-13
> 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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