[SI-LIST] Re: Fw: Question on Differential S parameters
- From: <mark_johnson@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>, <si-list-bounce@xxxxxxxxxxxxx>
- Date: Tue, 5 Apr 2011 08:51:39 -0600
One of the reasons that most TDR's and VNA's have operated in single-ended mode
traditionally is that it's easier to build a single-ended stimulus that is
identical no matter which port it is stimulating. When using a differential
stimulus (step or sinusoidal) the stimuli must be perfectly asymmetric
otherwise errors will be introduced. Why bother with the extra difficulty if
your device is Linear Time Invariant.
If the device to be measured is non-linear then just producing a perfectly
asymmetric differential stimulus may not be the only issue to worry about. For
example non-linear devices produce a different output dependent on the
amplitude of the stimulus. A differential stimulus (even if perfect) only tells
you part of the story.
Mark.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of wolfgang.maichen@xxxxxxxxxxxx
Sent: Monday, April 04, 2011 7:00 PM
To: Peter.Pupalaikis@xxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Fw: Question on Differential S parameters
Pete,
actually there _are_ VNAs that can perform truly differential measurements, but
that is indeed a fairly recent development (just about 3 years old).
E.g. Rohde & Schwarz ZVB Vector Network Analyzers which can go up to 20 GHz;
they have two independent sources (with calibrated phase of 1% accuracy - quite
a feat at these frequencies). I believe Agilent has similar offerings. That's
still quite a bit less in terms of max frequency compared to traditional
single-source VNAs.
There is nothing inherently wrong with using math to obtain differential
S-parameters from single ended measurements. As long as the system under test
is perfectly linear and time invariant this works beautifully, and there is
little to gain from using a differential stimulus (and it will be much more
expensive for the same maximum test frequency). So it's a good method e.g. for
traces on a printed circuit board, cables, connectors and similar passive,
non-magnetic structures. It starts getting "interesting"
though (read: prone to give completely invalid results) when you apply it to
the analysis of systems that are potentially nonlinear, e.g. an amplifier or
other circuits containing active elements or even just diodes.
Wolfgang
From: Peter.Pupalaikis@xxxxxxxxxx
To: si-list@xxxxxxxxxxxxx
Date: 04/04/2011 04:19 PM
Subject: [SI-LIST] Fw: Question on Differential S parameters
Sent by: si-list-bounce@xxxxxxxxxxxxx
From Pete Pupalaikis's Blackberry
----- Original Message -----
From: Peter J. Pupalaikis
Sent: 04/04/2011 05:54 PM EDT
To: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
Subject: Re: [SI-LIST] Question on Differential S parameters
Hi Charles:
Almost all "mixed-mode" measurements are made by almost all pieces of test
equipment as single-ended s-parameters that are converted to mixed-mode in
software. The only piece of equipment I know of that measures by driving
differential or common-mode signals is tektronix TDR and LeCroy WaveExpert
(also TDR). I'm not sure about Agilent TDR and to my knowledge all VNAs
measure single-ended with the mathematical conversion.
There are many ways to do the conversion - one is with formulas - another is by
cascading mixed-mode converter devices. Or they can be converted by driving
pairs of ports of single-ended models in simulation with proper terminations
and extracting components of interest.
Pete
From Pete Pupalaikis's Blackberry
----- Original Message -----
From: "Grasso, Charles" [Charles.Grasso@xxxxxxxxxxxx]
Sent: 04/04/2011 02:20 PM CST
To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Question on Differential S parameters
Hello all,
I have a question on how the SDD and SCD series of measurements are made.
Never having actually
performed these measurements I did a bit of reading and am unclear on the
process. What I understand is (at least from one manufacturer of test
equipment) that the 4 x 4 single-ended S-parameters (S11 to S44) are measured.
A built-in function then converts them to the mixed mode S-parameters.
My question: For SDD11 are the trace pairs driven with a differential signal
(in the software?).
By the same token for the SDC measurements are the traces driven with a common
signal?
Thanks!
Best Regards
Charles Grasso
Compliance Engineer
Echostar Communications
(w) 303-706-5467
(c) 303-204-2974
(t) 3032042974@xxxxxxxxx<mailto:3032042974@xxxxxxxxx>
(e) charles.grasso@xxxxxxxxxxxx<mailto:charles.grasso@xxxxxxxxxxxx>
(e2) chasgrasso@xxxxxxxxx<mailto:chasgrasso@xxxxxxxxx>
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