Wes,

The file reports an error. Can you load it?

Thanks very much. Dan

Wes Stewart wrote:
On Tue, 06 Dec 2005 16:48:32 GMT, "Frank"
wrote:


Sort of, but not exactly. As I described in another thread, the
pitfall is the fact that the phase lengths of the coupler are
different. You could set the phase offset to zero at frequency X but
when you change to frequency Y, there is a built-in error.

I should have added that you don't do this with a 50 ohm load, but a
short circuit instead. You want a big reflection with a known phase
for a calibration standard.

The first step is to establish the accuracy of the test equipment. A
short/open at the output port of the directional coupler should produce a
frequency independant phase shift between the forward and reverse coupled
port.


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.



If you are not getting 180/0 deg, then there is something
fundamentally wrong with the test set up. Connecting both probes of the
8405A to the same source should establish if any errors exist in the vector
voltmeter. Using the BNC/probe adapters is essential to maintain
repeatability in your measurements.

Reference HP's 778D coupler at
http://cp.literature.agilent.com/lit.../5952-8133.pdf :

"
Impedance Measurements

The 778D is also well suited for measurements of impedance when used with

the Agilent 8405A vector voltmeter. The technique is described in
Application

Note 77-3, Measurement of Complex Impedance, available at your nearest

Agilent sales office. Again, a reflectometry technique is used. With the
vector

voltmeter, however, both magnitude and phase angle of the reflection
coefficient

can be measured. This setup is shown in Figure 3.

Data can be read from the two meters of the vector voltmeter and transferred

directly to a Smith Chart to provide impedance of such devices as antennas
or

other passive components.


"It is many years since I have seen AN77-3, but I seem unable to find a copy
on the web.

Frank


The pertinent pages are he

http://www.qsl.net/n7ws/Pages%20from%20AN77-3.pdf