[SI-LIST] Re: Common Mode Return Loss Measurements

Hi Ray,

Well, I waited long enough for someone more knowledgeable than I to =
respond.  They didn't (or not publicly, anyway), so at the risk of =
looking foolish, here I go.  My expertise in this particular area is =
limited, but here's my 2 cents...

I thought the Netseminar presentation ("RF Balanced Device =
Characterization - Part I", link below) given by Agilent was very good.  =
Perhaps it answers your question.  I don't think they address your 2nd =
question specifically (I don't recall them using the term "even mode", =
and I haven't heard it in this context).

I believe "CMRL" and "differential S11" are too ambiguous to be =
meaningful.  As I understand it (from page 55 of the presentation), CMRL =
implies only that you're exciting your DUT with some common-mode signal =
and measuring some (undefined) return loss.  With that excitation, you =
have the option of measuring common or differential-mode return losses =
(or common or differential-mode insertion losses).  You CAN also excite =
the DUT with a differential signal and measure its differential or =
common-mode responses (return or insertion losses).  Measuring the =
common-mode insertion loss to a differential excitation might be useful, =
for instance, to determine how much of an ideal differential signal gets =
converted into common-mode voltage by your DUT (by mismatch of a =
differential pair, for instance).

I believe we're forced to be more specific (than "CMRL", for instance), =
using the SDD12, SCD21, etc. terms outlined in the presentation.

I think the most prevalent methodology is to excite single-endedly =
(which has both common and differential components) and, using math, =
extract the various flavors of common and differential mode responses.  =
You can do this with a typical 2-port VNA, but have to manually move =
your connections around and terminate unused ports until all ports are =
measured.  Multi-port VNA's do the moving and terminating for you, but =
the math is identical.

And FYI, if you use Agilent's "Differential TDR" you never excite the =
differential pair "differentially".  You are exciting each of the 2 =
ports single-endedly, measuring the responses, and mathematically =
deriving the differential response.=20

http://www.netseminar.com/nss/showSeminar?sem_num=3D802&branding=3DNSS&cl=
ientID=3DNSS

Jeff Loyer

-----Original Message-----
From: Ray Anderson [mailto:Raymond.Anderson@xxxxxxx]
Sent: Monday, April 21, 2003 3:47 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Common Mode Return Loss Measurements




A couple seemingly simple questions:

1)      What is the usually accepted method for measuring
        common mode return loss  (CMRL) ? (VNA with a balun,=20
        single ended VNA and math, differential TDR, ????).
        Would a CMRL measurement be the same as a differential
        S11 measurement?
=09
2)      Any practical differences between "common mode return loss"
        and "even mode return loss"? (doesn't common mode imply
        equal amplitudes and same phase while even mode only implies=20
        same phase ?)
=09
-Ray Anderson
Sun Microsystems

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