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
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