"Wes Stewart" wrote in message
...
On Tue, 06 Dec 2005 19:18:50 GMT, Owen Duffy wrote:

On Tue, 06 Dec 2005 11:37:04 -0700, Wes Stewart
wrote:


Maybe I misunderstand, but what you seem to be saying is that if I put
Probe A at point X on a transmission line and Probe B at point X+Y
(Y0), the phase difference with respect to frequency will not change.

The directional coupler is no different, it is sampling the main line
at two physically different locations.

With respect, I think you two guys are talking about different kinds
of couplers.

...Owen's coupler location discussion...
May be, but his link:

http://cp.literature.agilent.com/lit.../5952-8133.pdf

shows exactly what I'm talking about in Figure 3. Note the line
stretcher on one sample port. Because I haven't seen the inside of one
of these I don't know where the coupled arms reside with respect to
each other but the fact that H-P shows the line stretcher tells me
that they must have some (unavoidable, if not purposeful) asymmetry.
They claim 4 degree phase tracking but it's unclear to me whether they
mean the coupled arms with respect to the main line or to each other.


I have done this type of measurement and the coupler is somewhat
un-important in regard to where on the line its samples are taken from. The
line stretcher does the compensation to place both samples at the same
point on the main line. It is set in a calibration process to get to that
point (seems to mee only a short is required). Memory is starting to fade
here, but if you want the samples to be "at the plane of the DUT" then you
either must use two line stretchers or use the vector voltmeter capability
to compensate for the difference in sample location vs. DUT location. If I
recall correctly, this is simply a phase offset and the old HP vector
voltmeter has such an offset capability.

Look at it this way. You want the FWD and REF phases to be at the DUT. To
do this you must put an equal (electrical) length of line in all three
sections of line - the desired, the FWD sample and the reverse sample. I
admit the reflected part has an intuitive glitch that I can't resolve at
this time since it is always longer, but I know this works. You can sweep
the set-up and the phase of the two samples will sit right there on that of
a short . This rells you that the samples are "at the DUT".

Right??

73, Steve, K,9.D;C'I