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 and I will refine them as I move towards a final filter design. )
Interesting data Jack. For general info of all, ARRL uses -14 dBm
signals in their IMD test procedure. -14 dBm corresponds to two S9+59 dB
interfering signals using the standard 50 uV = S9 definition (-73 dBm).
>Bringing the discussion back to the K3, it shows the importance of
getting the roofing filter correct, as intermodulation generated in it
will come in "down the pipe" in the DSP stage.
Amen! Exactly why I want to see actual IMD results of the
various proposed K3 filters before making any decisions as to which I
order. I suspect Elecraft may see some surprising differences, even
as I type this, while preparing the IMD data we are waiting for.
>And, it's not just crystals that are non-linear passive devices.
Capacitors and inductors also have the same problem. In the case of a
capacitor, some dielectrics have a dielectric constant that is a
function of applied voltage. Ferrite and powdered iron (but to lesser
degree than ferrites) are also non-linear. Hence any component in the RF
path must be suspect and carefully evaluated for IMD performance.
This concerns me about the proposed variable bandwidth filters.
More components in the filter means more possibilities for unwanted IMD
product generation within the filter.
Happy 4th to all!
Bill W4ZV
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