[K3] [Elecraft] Checking crystal filters,

[AD6XY - Mike]

Traditionally this is a particularly hard measurement to make unless the
filter performance is very poor. 

Without professional test gear you will need to obtain or build two very
high quality signal sources, (say identical 7MHz crystal oscillators) at
least one of which needs to be tunable. These have to be buffered and then
very carefully combined. I think there was a design in the ARRL solid state
design book back in the 1980s. Then you can look for the 3rd order IMD
products at audio with something like Spectraview - hoping that your sound
card IMD performance is better than the radio, which it probably is not if
the radio is working. 

You need test sources with very low phase noise, more than 100dB down at the
spacing you are testing at. A typical signal generator is probably not good
enough for this measurement. Well designed crystal oscillators should be OK.

Maybe it is easier just to use the radio. When listening, if you can not
detect any intermods, then you don't have a problem with them.

Mike
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