I do not have the 8405a adapter kit or power splitter. I made "probe to bnc"
adapters with 3/4 inch copper plumbing stock. The threads on the back of the
probe are standard thread so it is easy to make an adapter and simply screw the
probes in. This allows the A and B inputs to be connected to the directional
coupler with bnc.

The measuring setup is a signal generator feeding a tee. One side of the tee
connects to the A channel input. The other side feeds the 'output side' of the
coupler. The B port is connected to the coupled signal port. The load is
connected to the coupler input. This should read the reflected signal.

The signal generator is adjusted to read 0 dbm on the A port for all readings.
(This is not possible for the 'shorted' readings). I do not have a pad between
the generator and the tee. However, the above setup appears to support
consistent readings.

Dan


Wes Stewart wrote:
On Sun, 01 Jan 2006 19:15:12 -0000, (Dave Platt)
wrote:


In article ,
Wes Stewart wrote:


The measuring setup is a standard signal generator feeding a tee. One branch
feeds the A input of the 8405A and the other a 20 db directional configured in
reverse, that is feeding the out port. The assumption is the reflected signal
will be read correctly on the in port. The in port is then terminated or
connected to an antenna.

It would also be nice it you had a 6 - 10 dB pad between the generator
and the directional coupler (DC); located right at the DC. You want
the source match to be set right there and the A probe to sample right
there.

The 8405A manual indicates the use of a power divider, and then a pair
of equal-value pads. One side goes to the probe T for the A
(reference) probe and thence to the termination, and the other goes to
the probe T for the B probe and thence to the device-under-test.


Actually, my manual does not show this. Although I have the full kit
of a resistive tee, two 50 ohm "N" sampling tees and appropriate
terminations, I don't believe Dan does.

The manual is quite clear that the A and B probes need to be connected
to points which are isolated from one another.


I've read this someplace, but again my version of the manual (unless
I'm really missing something) doesn't say it. Nevertheless, the
directional coupler provides the isolation between probes.


You really don't want
the oddities of the load connected to the B side to affect the
voltage/phase of the reference signal seen by the "A" probe - it'd
certainly wreck the measurement.


Using a power divider, and resistive pads for isolation is one way to
do this.


Actually, you do want to measure (include) the effects of the load on
the source. Although you ideally want the source to be unchangable
with respect to changes in the load (good source match). That is the
point of my suggestion to pad the source at the input to the coupler.
(Just as does paragraph 2.3 of Owen's reference)

But if the load -does- pull the source you damn sure want to know it
and account for it. The desired ratio is that between the incident
signal and the reflected signal. Sampling somewhere off in isolated
space via a tee and additional padding does -not- yield the incident
signal.


Using a pad followed by a dual directional coupler (as in
the experiment page to which Owen posted a link - thanks!) is another.


My point exactly. With the dual coupler the forward coupled arm -is-
measuring the incident signal -regardless- of what effects load pull
have on the source output.