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.

Wes, your earlier description makes is clear that samples in the
coupler you described are located at equidistant from their nearest
ends, but not in the centre, so at different positions in the coupler.
I think the coupled lines type of coupler for lower frequency use
might fit this category.

Frank assumes a coupler where the samples for both ports are taken at
the same physical location on the main line.

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 think a crossed
waveguide coupler might fit this category (depending on the way the
coupling holes are implemented).



Owen