[SI-LIST] Re: Assessing insertion loss profiles
- From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
- To: <buenos@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sat, 13 Jul 2013 15:41:29 -0700
Hi Istvan,
As Steve pointed out it depends. Here are a few cases.
If you have differential link routed with un-coupled or loosely coupled
traces, then it make sense to look at the single-ended S-parameters.
Obviously, all discontinuities in the reference conductors have to be
localized in this case, that technically equates such links into
single-ended.
If you have differential link routed with strongly coupled traces, then
looking a the single-ended S-parameters may be not helpful. The voltage
coupling coefficient for a quarter wave segment may provide a definition and
threshold to distinguish loose and strong coupling. Do a simple experiment.
Take a segment of 100-Ohm differential microstrip line (no vias, no
discontinuities) and compute S-parameters of a segment of such line with
different coupling coefficient (maintaining 100 Ohm). |Sd1d2| will look
clean in this case - no resonances. Then look at the magnitude of
single-ended S-parameters and you will see resonances in the single-ended
transmission in case of coupled micro-strips. This is because of coupling
and perfectly normal and will not cause any problems neither for
differential signal transmission nor for EMI.
Another case is a link with differential vias that are not localized for the
common mode (that is usually bad idea or legacy design). Both single-ended
and common-mode S-parameters will strongly depend on how the problem have
been truncated in the electromagnetic analysis (usually there are multiple
choices that produce very different results and have nothing to do with the
realigy). Though, the differential mode S-parameters may be still usable
(assuming that the differential mode is localized up to some frequency).
Best regards,
Yuriy
Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
3030 S Torrey Pines Dr. Las Vegas, NV 89146, USA
Office +1-702-876-2882; Fax +1-702-482-7903
Cell +1-206-409-2368
Skype: Shlepnev
www.simberian.com
Simbeor - Accurate, Fast, Easy and Affordable Electromagnetic Signal
Integrity Software
2010 and 2011 DesignVision Award Winner
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Istvan Nagy
Sent: Saturday, July 13, 2013 10:56 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Assessing insertion loss profiles
Assessing insertion loss profiles
Hi,
Sometimes I have to look at insertion loss profiles (before running a full
channel simulation with IBIS AMI models) to assess the quality of a channel
on a PCB.
Someone says we have to look at the mixed mode Sdd21 for this purpose, but I
think we should look at the S21 single-ended curve. The PCB-EM simulations
are done with single ended ports.
The Sdd21 looks a lot better than the S21, resonances look smaller.
I think Sdd21 would only be indicative if the channel and the return path
was perfect without discontinuities.
I have doubts about the signals being perfectly differential at both TX and
RX buffers. (for example 10GBASE-KR from an FPGA) So, what is the correct
approach?
Regards,
Istvan Nagy
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