[TenTec] Roofing filters

[Ken Brown]

Hi Ron,

The term "roofing filter" is sort of a reference to the fact that it is 
the first relatively narrow filter in the receiver filter. There may be 
bandpass filters in the front end that let through hundreds of kilohertz 
or even several megahertz sections of the spectrum. As you change bands, 
different frontend bandpass filters are used. The roofing filter however 
is in the IF and is at a fixed center frequency. They are typically a 
few kilohertz to a few tens of kilohertz wide. They are also typically 
not selectable, but rather a single filter that is in the siganl path no 
matter what. All of the following stages only have to deal with signals 
that get through the roofing filter. If the rig is a multimode rig that 
includes FM as one of it's modes, then the roofing filter will be wide 
enough (say 15 kHz) to let a narrowband FM signal through. So even when 
you are operating SSB or CW, the IF stages between the roofing filter 
and any later filters have to handle a wide bandwidth of signals without 
distortion, or there will be problems. Of course there are exceptions. 
Some multimode radios with FM have a separate IF and detector just for 
the FM, and may have a narrower roofing filter in the IF used for the 
other modes.

Ken N6KB