[time-nuts] Reverse isolation

Didier Juges didier at cox.net
Sun Mar 8 19:05:40 EDT 2009


As long as one is not trying to measure extremely low signals (or extremely
high isolation), the conventional method works, and I have used it many
times, I am not sure that the method would scale when dealing with very high
levels of isolation.

I certainly would not trust a conventional VNA for isolation of 100dB or
more, simply because such isolation is difficult to achieve in an instrument
that is supposed to be able to switch its source and receivers between the
two ports.

The HP 8722D which we use at work is only specified with 100dB dynamic
range, and I am not sure how that applies to reverse isolation.

Using a slightly offset test frequency while driving the device at its
normal operating frequency would only work when using a spectrum analyzer as
receiver. Most VNAs don't have the capability of rejecting large signals
close in, and separating the frequencies too much would make the test
invalid with a narrow band amplifier. I have used that method to measure the
hot output VSWR of a TWT amplifier for instance (that was interesting). Of
course, the spectrum analyzer does not give you the phase, but it's better
than nothing.

Didier KO4BB

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of John Day
> Sent: Sunday, March 08, 2009 5:47 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Reverse isolation
> 
> At 06:14 PM 3/8/2009, Didier wrote:
> >
> >This question is directed at Bruce, but if anyone else has a 
> >contribution, feel free to speak.
> >
> >Bruce,
> >
> >What is the best way to measure the reverse isolation of an 
> amplifier 
> >(particularly a buffer amplifier for a 10 MHz reference), when it is 
> >expected to be in the order of 100 dB or more?
> >
> >Feeding the output with a known signal and measuring at the 
> input with 
> >a spectrum analyzer comes to mind, but I am sure there must be 
> >something wrong with that technique, it sounds too simple.
> 
> No, not really. How else would you measure it? Reverse 
> isolation is basically the reverse gain - S12 - of the 
> amplifier. How does a VNA measure S12? Essentially inject a  
> signal at port 2 and see how much comes out of port 1.
> 
> If you want to get so picky as to determine S12 with a signal 
> in the forward direction then you have a problem. Because the 
> forward gain -
> S21 - is going to effectively swamp the signal going the other way. 
> So this is almost impossible to measure if the signals are at 
> the same frequency. In this case measure the S-parameters of 
> the amplifier in its 'normal ' configuration, then de-embed 
> the S-parameters of the device. Assuming the device is the 
> only non-linear element you are dealing with then from the 
> S12 & S22 values you can also figure out how the device 
> reacts in the reverse direction.
> 
> Then if you don't mind solving a large matrix you can figure 
> out how the device might react to passing a signal in both 
> directions. The reality is however that if the device is 
> within its linear range, which it is likely to be if you want 
> to distribute a reference or some such, the reverse behaviour 
> of the well terminated amplifier will approach the nominal 
> S12 value. The difficult part is to determine what happens 
> when the amplifier is not nicely terminated!
> 
> John
> 
> 
> >The presence of a signal at the input (or not) may affect 
> the operating 
> >point of the amplifier, so measuring from output to input 
> without such 
> >signal may not give a true result.
> >
> >Didier
> >
> >
> >
> >
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