Crystal filters are nonlinear passive devices by natureof the characteristics
of the crystal(s). The motional characteristic ofa quartz crystal is a
function of the level of the signal applied to it, knownas drive level
dependence or (DLD) in engineering terms. Crystal filters can
contributeinter-modulation when a strong out-of-pass-band signal is present,
whether theundesired signal is immediately adjacent to the pass band frequency
or removedsome distance in frequency from it.
Another concern of placing a narrow bandwidth filter inthe RX input is the
effect it has upon the receiver impulse noiseblanker. A narrow bandwidth
filter stretches the noise pulse bandwidth bymany orders of magnitude and can
render the NB ineffective and contributes IMDproducts. For this reason, in
modern day receivers, the noise blankersampling/processing is placed ahead of
the narrow band IF filters.
Placing a narrow bandwidth filter atthe input of the RX will cause IMD if the
NB is in use.
In a contest operationthe input filter will be subjected to MULTIPLE nearby
strong signals that arenot in the pass band of the filter. This condition is
accentuated in amulti-multi operation.
Dave, K1FKFort Kent, ME
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