FreeLists / si-list / [SI-LIST] Re: PureMode VNA vs 4port VNA]

["Brad Cole" <cole@xxxxxxxx>]

Ray,

As you may know, the concept of mixed-mode s-parameters was first published
by Prof. Bob Eisenstadt of the University of Florida and Dave Bockelman of
Motorola. It was first commercialized by ATN Microwave, which was later
acquired by Agilent Technologies.

Eisenstadt and Bockelman claimed that there were advantages to the pure-mode
system which actually applies a differential stimulus and a common stimulus
to the device-under-test. The argument was that the isolation from one pair
to the other (input to output) was very low for a differential stimulus
because of cancellation that occurs due to the symmetry of the signals. This
was thought to be particularly true for RF probing configurations. There is
no benefit claimed for common signals.

ATN experimented with this extensively and reached a different conclusion.
The reason is basically that to generate the differential and common signals
to stimulate the device requires some circuitry. However, hardware is never
ideal, and even though the errors that it introduces (phase imbalance,
amplitude imbalance, differential & common matches) can be corrected to some
degree, the results are not as good as the mathematically perfect balun that
is effectively used with the single-ended counterpart.

ATN built both types of systems. Even using coaxial calibration standards
and DUT's, the single-ended system consistently outperformed the pure-mode
system because of the reason I described above, especially for the low-level
parameters such as the mode-conversion terms. In the end, the added hardware
complexity and more time-consuming calibration procedure did not buy any
performance improvement.

Of course the underlying assumption here is that the DUT is linear. It can
be active or passive, reciprocal or non-reciprocal, as long as it is linear.
Even single-ended s-parameters are linear by definition, though, and for
interconnect characterization this is a good assumption.

Brad

> 
> From: Ray Anderson <ray.anderson@xxxxxxxxxx>
> Date: 2005/04/04 Mon PM 03:23:05 CDT
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] PureMode VNA vs 4port VNA
> 
> Over the past year or so there has been a lot of discussion of 
> mixed-mode s-parameters, why they are useful, conversion of single-ended 
> to mixed mode, etc., etc....
> 
> As I understand it, mixed-mode s-parameters can be mesasured with a true 
> pure-mode VNA, with a 4-port VNA,  or with a TDR (the last 2 methods 
> requiring mathematical post-processing of the measured data to create 
> mixed-mode s-parameter data). This post processing can occur either in 
> the instrument (with appropriate firmware) or after the fact in a 
> separate compute environment.
> 
> Is it true that a true pure-mode VNA is still not available commercially 
> from any of the test equipment vendors? I wonder if anyone knowledgable 
> on the topic can comment in the practical differences a user should be 
> aware of  when utilizing a 4-port VNA to measure mixed mode s-parameters 
> as opposed to the fabled pure-mode instrument.
> 
> -Ray
> 
> -- 
> Raymond Anderson
> Senior Signal Integrity Staff Engineer
> Product Technology Dept.
> Package Engineering Group
> Xilinx Inc.
> 
> 
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