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

[Dave Bowker]

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