On Tue, 06 Dec 2005 23:13:35 GMT, Owen Duffy wrote:

On Tue, 06 Dec 2005 14:22:31 -0700, Wes Stewart
wrote:


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 could be wrong, but I think that 4 deg seems part of the tolerance
of the phase alignment of the two sample ports wrt each other over the
frequency range.

I tend to agree, but as someone who used to write specs and then sit
back and watch vendors try to skirt them, I'm always dubious. :-)

Perhaps this is a coupler where the ports are
approximately in phase, and perhaps the line strether is to adjust
phase of forward and reflected ports to create a new reference plane
where it is needed or convenient to the measurement.

I think what's being missed in this discussion is this: The 8405 has
a phase offset adjustment that can make -any- relative phase read 180
degrees on the meter. So you could put a short on 100 feet (30.48m)
and adjust the phase offset to make the meter read 180 deg. This
would be fine until you changed frequency then you're in trouble.
That same effect is the reason for adding a line stretcher; it's to
make that 180 degree relationship track with respect to frequency.

Paragraph 2 of the document I provided explains this nicely.

I've built reflectometers (many times) where dual couplers were not
available and two back-to-back singles were used. (Sometimes, three
were used with the added one in a feedback loop used to improve the
source match of the generator) I can guarantee you that the two
coupled arms didn't phase track and that is the general case that I
was trying to put forth at the outset of this discussion.