Crystal filters are nonlinear passive devices by nature of the characteristics
of the crystal(s). The motional characteristic of a quartz crystal is a
function of the level of the signal applied to it, known as drive level
dependence or (DLD) in engineering terms. Crystal filters can contribute
inter-modulation when a strong out-of-pass-band signal is present, whether the
undesired signal is immediately adjacent to the pass band frequency or removed
some distance in frequency from it.
Another concern of placing a narrow bandwidth filter in the RX input is the
effect it has upon the receiver impulse noise blanker. A narrow bandwidth
filter stretches the noise pulse bandwidth by many orders of magnitude and can
render the NB ineffective and contributes IMD products. For this reason, in
modern day receivers, the noise blanker sampling/processing is placed ahead of
the narrow band IF filters.
Placing a narrow bandwidth filter at the input of the RX will cause IMD if the
NB is in use.
In a contest operation the input filter will be subjected to MULTIPLE nearby
strong signals that are not in the pass band of the filter. This condition is
accentuated in a multi-multi operation.
Dave, K1FK
Fort Kent, ME
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