[time-nuts] Reverse isolation

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Mar 9 00:58:05 UTC 2009

Lux, James P wrote:
>> 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.
> Jim
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.

Its even more challenging when the isolation amplifier has a nominal
gain of 0dB coupled with a reverse isolation of 120dB or more.
In this case one would be looking at measuring phase shifts on the order
of 1urad or less.

In this case one could try applying say a +10dBm signal to the input, 
modulate the load reflection coefficient and look for a modulation
frequency related sideband at the input port.


More information about the time-nuts mailing list