I've re-run the crystal filter intermodulaton tests this morning, with 2
KHz generator spacing. This allows the IMD products to fall within the
filter passband, and gives a more useful picture of the total filter
performance.
Details and plots are at
K8ZOA:
Monolithic crystal filters are known for IMD, and the test was run with
a rather high drive level, around +9 dBm. However, the data is quite
instructive and should convince anyone who doubted that crystal filters
could themselves be a source of IMD. (These measurements are quick and
dirty
I've mentioned before that crystal filters are not perfectly linear
passive devices and can in fact generate intermodulation products. The
reason is that a crystal's motional parameters are not constants, but
rather are a function of the applied drive, something called drive
level dependency
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