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

Hello, guys,

We have had great success measuring differential and common insertion and
return loss using differential and common mode TDR.  Jeff is correct in
stating that with an Agilent TDR you do not really get a true differential,
it is done through math, whereas with Tektronix it is a true differential
(two signals shooting at the same time). However, if you are working on
interconnects, which are linear, that does not matter. Good S-parameter data
can be obtained up to 12 Ghz, and beyond with good fixturing, probing and
cabling.

The TDR measurement approach to S-parameters it works great when you are
trying to meet a 26dB crosstalk digital specification - TDR provides more
than sufficient dynamic range. However, if you are measuring a microwave
filter, where all 100 dB of dynamic range provided by a VNA is still
required, I would recommend a VNA.

We have re-worked our tutorial on S-parameter measurements from TDR, and if
anyone is interested, please download it from our web site,
http://www.tdasystems.com/support/usermaterials/tutorials.htm


Thanks,

===================
Dima Smolyansky
TDA Systems, Inc.
4000 Kruse Way Place
Blvd. 2, Suite 300
Lake Oswego, OR 97035
(503) 246-2272
(503) 246-2282 (fax)

Download new IConnect 3.0 Quick Tour at www.tdasystems.com/products/  !

----- Original Message -----
From: "Charles Hill" <chuckh@xxxxxxxxxxx>
To: <jeff.loyer@xxxxxxxxx>; <Raymond.Anderson@xxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, April 22, 2003 5:41 AM
Subject: [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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