[time-nuts] Reverse isolation

Lux, James P james.p.lux at jpl.nasa.gov
Sun Mar 8 23:41:17 UTC 2009

> Actually, it depends on the distance from the hot carrier, the amplitude
> of the hot carrier and the particular VNA in question. My VNA allows me
> to change filters as well as integration time.

And especially, where is the selectivity in the VNA's receiver?  You might
have a 10 Hz filter, but if the receiver's front end is wide open, then the
dynamic range requirement might be a challenge to meet.

Consider a 30 dB gain amplifier claiming 120dB reverse isolation..

You put in -30dBm and get 0dBm out in the forward direction.  Meanwhile, you
inject 0dBm into the output of the amplifier (from the VNA) and you expect
to see -120dBm appearing at the input.  Depending on how good the
directional coupler/bridge is that feed the port 1 reflected power receiver,
you might be facing measuring a -120dBm signal next to a -30dBm signal,
which is challenging...

Assuming the DUT is linear (hah!) one could just apply load with different
reflection coefficients to it when measuring S11.  This might even be
harder, though... With -120dBm reverse isolation, you're basically looking
at seeing the effect of adding a coherent signal 90 dB down to the signal
you're already measuring for S11. That's not going to result in a huge
change in amplitude or phase (e.g. About 1 part in 1E4 or 1E5)..

Maybe for this list, though.. Measuring phase shifts of 1E-5 radian.. Walk
in the park, as it were.


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