Jerry Flanders wrote on Tuesday, June 03, 2008 1:34 AM
I also have one of Jack Smith's Z-10000U buffer amplifiers (Clifton
Laboratories) running between the K3's KXV3 output and the SR40 for
isolation. (I didn't test to see if the isolation is really needed - maybe
not, since the LO is offset 24 KHz.)
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Jerry, in my opinion it is always wise to use a buffer amplifier which
exhibits a high degree of reverse isolation, such as Jack's Z-10000, when
picking off a sample of signals from a receiver's signal path. As I am not
too familiar with the K3 I could be wrong but I believe that the point at
which you are taking a sample for the SR40 is somewhere between the first
mixer and the input of the 8.215 MHz IF filter. If that is the case and you
did *not* use a suitable buffer amp, the SR40's LO 'leaked' signal would
most likely appear at the input of the filter even though its offset is 24
kHz. This means that the filter has to deal with more unwanted signal power
than just the 'routine' stuff coming from the mixer. Even worse is the
possibilty that the SR40's LO 'leaked' signal might back up into the mixer,
mix with the LO or its harmonics and/ or other signals in the mixer, and
produce its own family of spurious products which would cause additional
problems for the filter. Some of these products might even lie within the
filter's passband and be heard, but I have not done the maths to find out if
that is the case.
As an aside, a crystal filter can itself generate significant IMD products
if overdriven, which is why it it is essential to measure the IIP3 of a
crystal filter at various input levels and spacings when designing a
receiver.
73,
Geoff
GM4ESD
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