[TenTec] Explanations of the Different Types of Filtering on Radios

[Stuart Rohre]

I think the easy way to think of it is to realize the ideal filter is a 
rectangular pass band of frequencies, ie the 455 kHz traditional 
superhet filter was responding to a wider band than that one frequency. 
It is just centered at 455 KHz.  (Or 9 MHz, or whatever).

Sometimes, interference will appear just inside the pass band of the IF, 
whatever the center frequency of the filter.  If you can alter the 
tuning of the IF, you may push the interference off the shoulder of the 
response of the filter, and thus eliminate its QRM affecting what you 
are listening to; at a different point on the filter response.

A notch filter on the other hand, reduces the radios response by a 
tunable notch that sucks out amplitude at the offending signal's 
frequency.  It operates inside the passband of the IF, if an IF notch type.

RIT, changes by a small amount or vernier, the main tuning of the radio, 
to again clarify the desired signal vs. others near by.  By tuning off 
one direction or the other, you may get a usable signal from the QRM.

Basically, all the means attempt to separate desired signals from others 
or noise.

Hope that helps.
Stuart Rohre