[time-nuts] Reverse isolation
djl at montana.com
Mon Mar 9 01:47:42 UTC 2009
Best isolation is via a piece of fiber optics.
> Lux, James P wrote:
>>> Actually, it depends on the distance from the hot carrier, the
>>> 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
>> have a 10 Hz filter, but if the receiver's front end is wide open, then
>> 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,
>> inject 0dBm into the output of the amplifier (from the VNA) and you
>> to see -120dBm appearing at the input. Depending on how good the
>> directional coupler/bridge is that feed the port 1 reflected power
>> 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
>> reflection coefficients to it when measuring S11. This might even be
>> harder, though... With -120dBm reverse isolation, you're basically
>> at seeing the effect of adding a coherent signal 90 dB down to the
>> 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..
>> in the park, as it were.
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> 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.
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Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
Huson, MT, 59846
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