Since the instability only
occurs in one frequency band, I suspect the signal
generator first. Have you tried measuring the signal
generator directly without the coupler (use a 20
dB pad if power is too high) to see if it is stable
at 1785 MHz? Put the power meter as close to the
generator output as possible to bypass all cables
and connectors in-between.
If the signal
generator power is stable without the coupler and
cables, then next thing to suspect would be a bad
cable or connector (or connection). It is possible
for a pinched or partially broken cable (intermittent
strand contact) to cause problems in a narrow band.
Poor connector contacts can do the same. Connect
your power meter at the end of the cable where the
coupler normally goes, and use the attenuator if
necessary, and check for instability. If it is stable,
shake and twist the cables a little bit to see if
it becomes unstable.
If the signal is still
stable, then the only thing left is that the couple
is bad. If it was hit with too much power at some
time in the past, there could be a burn inside that
is selectively unstable. One way a power coupler
can be over-powered is to crank a lot of power into
it with the output port either open (more likely)
or shorted. Doing so causes the coupler structure
to have to dissipate much more power than it is
designed to handle.
Let me know the outcome
of these test, if you try them, please.
- Kirt Blattenberger