Owen,

That idea seems to work. I set it up and then looked and the 'interference', the
change in phase based on changing the pad. Zero pad showed several degrees phase
shift from -40 dbm, 10 dbm showed small shift, and 20, 30, and 40 were all about
equal. I decided on 20 dbm as a practical base.


Thanks - Dan


Owen Duffy wrote:
On Sun, 01 Jan 2006 16:51:37 -0800, dansawyeror
wrote:


Owen,

Yes, you are right. The single coupler doesn't isolate the channels. Putting a
10 dbm attenuator between the tee and the coupler changes the 50 Ohm reading. I
think a dual directional coupler is required. I will have to put this on hold
until that problem is solved.



Dan, it seems to me that you should be able to make measurements with
a single directional coupler (DC)

You could connect your signal generator to the directional coupler via
a 40dB attenuator, and put the chan A probe T on the sig gen end of
the attenuator. This sample should be fairly independent of the
reflection from the unknown load (to the extent of the 40dB
attenuator), and so approximately proportional to the incident wave
alone.

The sample from the DC "reflected" port (properly terminated) is
fairly independent of the incident wave (depending on the F/B ratio of
the coupler) and so is approximately proportional to the reflected
wave alone.

Calibration of the B channel magnitude with a s/c and o/c taken as
rho=1 provides the basis for measurement of Gamma. The angle of Gamma
should be calibrated to 180 and o deg respectively.

BTW, the angle of Gamma for a 50 ohm termination is unimportant if rho
is very small. The angle of Gamma is real important for s/c and o/c
and ought be almost exactly 180 deg difference (if not, you have a
instrument problem).

Following this procedure, if the magnitude of the B channel on the
unknown load measures for example 9.5dB below the B chan magnitude on
a s/c, then the return loss is 9.5dB and the VSWR is 2:1. rho (the
magnitude of Gamma) is 0.333 and you could measure the phase offset
from the o/c angle to determine the angle of Gamma.

Why won't this work?

Owen
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