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

Hi Ray,

The method I use to measure common mode return loss is a single ended VNA
plus math.  Having been through the math, there are a few comments I have to
add to the discussion.

The math as well as the calibrations both have levels of correction which
affect the overall accuracy of the result.  A full set of math involves a
fair amount of calculations.  And you already know VNA calibrations have
many variations yielding different accuracies.

The formulas used by the multiport VNAs are similar for the most part to a
2-port VNA measurement set except the multiport must account for test set
switches.  The 2-port ignores the repeatability of the connectors.

I've found differing definitions of "even mode" and "common mode" and so
prefer to just measure the multiport S matrix, then get the exact
mathematical definition of the quantity desired, including the impedance
level.

The issue with true differential excitation is known calibration standards.
An additional issue with balun measurements is establishing the reference
plane at the DUT.

I hope this helps.

Regards,
Chuck


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Loyer, Jeff
Sent: Monday, April 21, 2003 7:29 PM
To: Raymond.Anderson@xxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [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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