[time-nuts] Reverse isolation

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Mar 8 22:28:30 UTC 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.
> 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

Network analysers like the Agilent E5071C can be limited by the fixture
used, to around S22 (fixture) ~ -120dB@ 10MHz.

In principle one just feeds a signal into the output and measures the
resultant signal that appears at the input.
However when attempting to measure reverse isolation of 120dB or more 
cable leakage, leakage from connectors (non screw mount connectors like
BNC can be quite leaky) need to be considered. Also if one isn't careful
with the grounding system this can limit the measured attenuation.
Leakage from the test source also needs to be considered.
The HP journal article on achieving 120dB attenuation with an attenuator
is informative on some of the issues involved.

The other major consideration is that unless one is making the
measurements within a shielded room the ambient RF signals may make such
measurements difficult.


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