[time-nuts] Reverse isolation

[Didier]

But not necessarily lowest noise

Didier 

> -----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.
> Don
> 
> 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. 
>  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
> >>
> >>
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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
> POB 134
> Huson, MT, 59846
> VOX 406-626-4304
> www.lightningforensics.com
> www.sixmilesystems.com
> 
> 
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