[SI-LIST] Re: Common Mode Return Loss Measurements
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <Raymond.Anderson@xxxxxxx>
- Date: Mon, 21 Apr 2003 18:28:40 -0700
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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