In article ,
Wes Stewart wrote:

Bingo. Didn't seem like an "Electrical feature" to me :-)

Yeah. I'm not sure just why there would be substantial interaction if
the two points are connected to the same test point, since the rated
impedance is pretty high even without 10:1 isolators.

But I still submit that when you separate the bridge, insert a DC and
some cabling, you lose the symmetry and the signal measured by the A
probe is not necessarily the same as the signal incident at the input
to the DC. Close maybe, but not something I would rely on.

The signal on the other side of the T-and-attenuator setup wouldn't be
the same as the signal at the input to the DC, certainly, since the
signal at the input of the DC would be affected by the reflected
signal. I don't disagree with you there.

What I suggest, though, is that the signal on the "A" probe (at the
other side of the T from the DC), and a signal as seen at the output
of the DC's "forward" coupler line, ought to be very closely
correlated. They'd differ by the coupler's coupling factor, of
course, and there's be a bit of phase shift from the coupler
(dependent on the coupler line length and the frequency). However,
the loading at the coupler output from the load (or the calibration
short) ought not to affect the signal appearing at the 'forward' tap
on the coupler.

Remember, when doing the calibration there is a 100% reflection. This
can have a huge perturbing effect on the incident signal at the
coupler input if the source is not well matched.

Agreed, and I don't suggest that measuring the incident at the coupler
input is a good idea.


That's why I
originally suggested a pad right at the coupler input, especially if
there is some cabling between the generator (or power splitting tee)
and the DC.

Agreed.

No. The B probe, in the single directional coupler arrangement, is
not measuring -incident-, but reflected signal.

True. I was assuming a double directional coupler, and asserting that
the "forward" output on the coupler will produce a signal equivalent
(except for scaling and perhaps a tad of phase shift) to a signal
taken from the far side of the splitter-and-pads "T".

In any event, Dan has stated that he doesn't have all of this stuff
and is stuck using the DC only. My suggestion holds, put a pad at the
DC input, measure the incident at the DC input and of course, the
reflected at the coupled port.

Yes, that should work quite well, and I think it'd give results pretty
much equivalent to [1] a dual directional coupler or [2] the
splitter-and-two-pads isolation arrangement.

--
Dave Platt AE6EO
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