[SI-LIST] Re: Relevance of Common Mode Return Loss
- From: Jory McKinley <jory_mckinley@xxxxxxxxx>
- To: Eric Bogatin <eric@xxxxxxxxxxxxxxx>, David Instone <dave.instone@xxxxxxxxxx>, olaney@xxxxxxxx
- Date: Tue, 20 May 2008 09:52:09 -0700 (PDT)
Yes (sort of in the first order) there is a spec on the amount of common mode
signal allowed for some serdes standards. The spec is given as a return loss
of x dB form some xfreq to some yfreq. So by making the common mode return
loss measurement with a reference impedance of 25ohms will give us the first
order S21 = sqrt(1-S11^2). Is this good enough?
By the way how would one measure the interconnect/package/on-chip circuitry to
evaluate the conversion of the differential signal into common signal (SCD21)?
-Jory
Guys-
If there is so much concern about the presence of the common signal on the
interconnect, and its rattling around, shouldn't there be a spec on the
amount of common signal allowed at the receiver?
Anyone have a feel for what a reasonable value is, before it starts to
affect the jitter, as Steve Weir pointed out?
Lynn Greene suggested that any common signal present could be converted back
to diff and screw up the diff signal. If the source of the comm. signal is
from asymmetries in the interconnect, then isn't this the first order
problem to fix?
Shouldn't there be a spec on the SCD21 performance of the interconnect to
evaluate the conversion of the differential signal into common signal? This
is the first order problem, the second order one being the conversion of
"rattling around" common signal back into differential signal, further
screwing up the diff signal.
Steve Weir suggests that a spec for the SCC11 and SCC21 of an interconnect
is related to the fact it is easy to do, not that it is the most reasonable
approach. It is sort of like the joke where the punch line is, "because the
light is better over here" (only if I am publicly encouraged will I provide
the rest of the joke)
I am still trying to understand the importance of the SCC11 and SCC21 spec,
as opposed to a spec on the magnitude of the common signal, or on SCD21 or
SCD11.
Does anyone have any insight on the discussions that went on at the
committee meetings for the specs?
Thanks
--eric
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of David Instone
Sent: Tuesday, May 20, 2008 4:16 AM
To: olaney@xxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Relevance of Common Mode Return Loss
Orin,
As Lynne pointed out in her posting on this subject, part of the common
mode energy can get converted to differential. Surely then, reflecting
the common mode back to the Tx, even if the Tx is a good CM match, would
give it twice the opportunity to interfere with the differential. How
much this matters of course depends on how imbalanced the differential
lines are and how imbalanced the signal is.
Centre tapping the differential terminating resistor, at the Rx, to
ground only fully terminates the common mode if there is little coupling
between the lines, if they are coupled then three resistors are
required, 1 from each line to gnd to terminate the even mode and 1
across the lines, which in parallel with the other two terminates the
odd mode, how necessary this is depends on how tightly coupled the
lines are. Currently SATA and PCIe implementations that I have seen
have the Rx termination inside the chip which makes the 3 resistor
termination difficult to achieve. This makes a common and differential
return loss specification at the Rx relevant as it enables the traces
and cable to be designed to match the termination in both modes.
Regards
Dave Instone
+44 (0)1235 824963
OXFORD SEMICONDUCTOR LIMITED
25 MILTON PARK
ABINGDON
OXFORDSHIRE
OX14 4SH
Registered in England no 2733820
Registered Address: As above
olaney@xxxxxxxx wrote:
> Since we don't want them, common mode signals can be treated
> differently than for differential mode. Given that common mode is
> undesirable, at the transmit end we often use a deliberate mismatch
> (CMC) to reflect this signal back to the transmitter. This energy can
> be absorbed by the transmitter if there is an adequate common mode
> backmatch, or it can be left to ring between the driver and choke if
> that is considered harmless. At the receiver, the intent of providing
> a common mode termination is simply to prevent unwanted CM energy from
> returning up the line, giving it an additional chance to radiate.
> If the common mode signal is terminated before the differential signal
> passes through a CMC to reach the DM termination at the receiver, then
> the best of both worlds is achieved: the CM signal is both absorbed
> and suppressed. The receiver common mode range becomes much harder to
> violate. For coding with a zero at DC (accepts AC coupling), a center
> tapped inductor is an easy way to provide the CM termination:
>
> Orin Laney
>
>
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