Wes,

I only have copies of a few pages from the app note showing a bi-directional
coupler. Is your reference that the note shows a bridge?

Thanks - Dan

Wes Stewart wrote:
On Sun, 01 Jan 2006 22:10:09 -0000, (Dave Platt)
wrote:


In article ,
Wes Stewart wrote:


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.

The BAMA copy mentions it in paragraph 3-14. Later text indicates
that attaching the two probes to a single point is an appropriate way
to set phase-zero.


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

Nevertheless, the
directional coupler provides the isolation between probes.

I see the issue, and I think I was conflating two different sorts of
measurement regimes.

The splitter/isolator/pad arrangement I was referring to appears on
page 3-3 of the 8405A manual available at BAMA. It's what's
appropriate for doing an in-line test of a transmission line or other
network, where you want to see the effect of the network itself and
can measure (via probe B) at the network's output.

Page 3-4 shows a somewhat similar hookup, which doesn't include the
resistive pads... I presume because the device-under-test (an
amplifier) is assumed to have high isolation as part of its design.


No, that's very similar to figure 11 in AN77-3 that I mentioned below.
Without inserting the device under test per Fig 3-3, but removing the
right hand 50 ohm load and connecting the device there, reflection
measurements can be performed. That's a technique described in
AN77-3. The configuration is that of a Wheatstone bridge as mentioned
earlier.

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.

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. 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.


Neither of these hookups wouldn't work for measuring an antenna, since
you can't measure at the antenna's output.


Not so, see above.


Instead, using a
directional coupler provides the necessary isolation, and (as you
point out) lets you determine the incident and reflected signals
accurately.


I've got to clarify this a bit if I can...

If you have the full set of parts per figure 11 in AN77-3 and you are
using them as shown, then with equal loads on the two ends, the
circuit is essentially a resistive Wheatstone bridge in balance with
the null detected by the difference between probes A and B.

In this case, the "incident" signal -is- measured by the A probe and
the effects to the source by a changing load are incorporated into the
measurement.

In the case at hand, at least as I imagine it, there is no longer an
nice tidy resistive Wheatstone bridge, but some cabling and a
directional coupler in the mix. In this case, the generator is no
longer the "source", the source is the signal at the input to the
coupler. It is my belief (unless I change my mind later) that a
sample derived from a resistive divider remote from the input to the
directional coupler is not a true measure of the incident signal.

Hmmm. In the general case, I believe you're correct.

I suspect that the setup shown in the 8405A manual sets up a specific
special case, though. The diagrams and text seem to be defining a
case in which:

- there is a physical and electrical symmetry in the T arrangement -
that is, the power splitter is symmetrical, and the pair of
attenuator pads between the splitter and the (A probe tap) and
(device under test) are matched. The manual makes a point of this
issue.

- The pads being used are matched to the system's transmission line
impedance, so that any reflected signal coming back from the
DUT/coupler sees a proper termination by the source (the pad and
signal generator, in this case) and is not re-reflected.

In this particular situation, I believe that the incident signals
reaching the DUT (the input to the coupler, in this case) and the "A"
probe, would be identical... would they not? The proper termination
of the reflected wave will mean that it won't re-reflect off of the
generator and alter the incident wave. The "A" probe signal (off on
its side of the "T") and a signal read out via the incident-wave tap
on the directional coupler ought to be the same, once the coupling
coefficient is taken into account... no?


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

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.