[time-nuts] Reverse isolation
didier at cox.net
Tue Mar 10 01:46:51 UTC 2009
But not necessarily lowest noise
> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Don Latham
> Sent: Sunday, March 08, 2009 8:48 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Reverse isolation
> Best isolation is via a piece of fiber optics.
> Bruce Griffiths
> > 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.
> >> 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
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> > Jim
> > 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.
> > Bruce
> > _______________________________________________
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> Dr. Don Latham AJ7LL
> Six Mile Systems LLP
> 17850 Six Mile Road
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