One thing everyone needs to keep in mind is IMD products at
one point can even cancel other IMD products produced at
another point in the system. You don't necessarily get the
worse case condition, you might get something that looks
better than it really is.
You need a way to drive the amplifier with a very clean
two-tone signal, and the detector has to be very clean.
Also there is a basic flaw with two-tone tests. Two-tone
tests don't vary the power supply loading like a voice
signal would. Flaws in grid voltage or high voltage dynamic
regulation may not show at all. Almost always the error
makes things look better than they really are.
As a general rule it would be impossible to get a good
reading with a standard amateur SSB transceiver as the
driver. You would have to use two radios on CW through
attenuators into a combiner, and for the combiner to work it
has to be terminated in 50 ohms (or whatever it is set for).
That combiner cannot be run into the amplifier under test,
it has to go through a VERY linear amp that has a constant
input impedance. That amp then drives the actual device
under test.
As for the detector, a good selective level meter will work.
That means you could press a receiver into use if you are
absolutely sure it is good enough at close spacing and if
you have a calibrated attenuator that has enough steps to
always keep the S meter at exactly the same level as you
check various IM products against the desired carriers.
If you mix to audio and use a sound card, how will you sort
out the opposite sideband of the mixer? How would you know
the IM level of the sound card with all the multiple signals
present? IM occurs a lot faster with multiple signals than
it does with just two pure tones.
73 Tom
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