On Sep 12, 9:22 am, "David" nospam@nospam wrote:
I have a RF switching unit that multiplexes 4 input RF ports to 12 output
ports. The unit carries RF of a frequency range from DC to 1 GHz. It carries
GPS signals and other RF from an antenna.

Can you advise me of the design of a test jig to accurately test this unit?
The test jig will contain a Network Analyser.

Where I have a split path or differential measurement, do I always have to
use phase matched and batch matched cables?

Does my test jig switch calibration loads between ports? The cal load being
an open, a short and a load.

Do I get the paths the same by using phase matched & batch matched cables,
and then do a vector gen to save a 'footprint' of the test jig when it is in
a known good
state?

How do I minimise errors introduced by imperfections in the test jig? I am
looking for the test jig to measure the absolute SWR and insertion loss of
the RF switching unit, using S parameters.

Modern vector network analyzers will provide for calibration at the
ends of cables, even if the cables go through relays or splitters or
whatever. The the cables (plus other components) introduce error only
to the extent that they are not perfectly constant and that they
introduce loss which decreases the signal-to-noise of the
measurement. Vector network analyzers commonly can do a full two-port
measurement in one setup; that is, they have the ability to measure
transmission and return loss in both directions, and that may save on
the number of setups required.

You didn't directly mention a desire to measure isolation. You didn't
mention if you want the tests to be fully automated, or run through
operator intervention: three possible scenarios are that an operator
connects a pair of cables from the VAN to certain ports on the DUT and
adds some terminations, runs a test, changes the setup, runs another
test, ...; an operator connects a complete set of cables from a test
jig which includes loads and path-switching relays and then initiates
the test sequence; or some machinery connects the DUT to the test jig
and runs the tests. In the first case, the "test jig" is very simple--
in the other two, it's considerably more complicated. You didn't
mention the accuracy you need to achieve. I'm a little puzzled by the
"DC to 1GHz" and the "GPS signals," in that GPS RF signals are in the
range above 1GHz.

We do testing of a somewhat similar nature here, using RF relays to
switch the test configuration for different tests. It's important to
calibrate such a system, and you need to establish how often the
calibration should happen. Because of the number of ports involved,
your calibration may be a bit tedious, so it could be an advantage to
insure that you use especially stable cables and other components so
the calibration doesn't have to be done too often. It's also
important to do a careful error analysis so you understand the source
of the errors and their expected magnitudes. At first look, I'm not
seeing that you need matched cables in your system, but you do need to
calibrate the net effect of the cables and whatever switching you use,
and account for the variability as cables are flexed and as relays
close with slightly different contact resistances each time. It's
possible that your accounting will tell you that those variabilities
are so far below your allowed tolerances as to be unimportant, but you
should think about them in any event.

You'll probably find it worthwhile to carefully document your thinking
about the design of the test system, so that if anyone asks in the
future you can show them just what went into the design.

Cheers,
Tom