[time-nuts] Reverse isolation

Magnus Danielson magnus at rubidium.dyndns.org
Sun Mar 8 22:48:23 UTC 2009

Didier skrev:
> 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.
> 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. 

If you are concerned about that issue, then you can apply a signal of a 
different frequency on the input and then use your network analyser to 
sweep the range. To be able to sweep the full range apply two different 
frequencies so you can make partial sweeps of the other range.

Otherwise the buffer amplifier should see the 50 Ohm termination of the 
network analysers as you run the buffer amplifier in reverse.

I suspect that isolational amplifiers often is analysed in sections.
If you have -120 dB reverse gain it can be a bit hard to measure.

Notice that a network analyser is automatically synchronous to the 
source, which makes detection of modulated frequency more sensitive.


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