Topband: "Thinking out loud" - Something else to Think About

[Dave Bowker]

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