I have received quite a few responses to my request for a clear
definition of the term "roofing filter". Most of the respondents seem to
think that they know exactly what a roofing filter is, yet they don't
all agree exactly with each other. The best (in my opinion) and most
thorough response was from Bill W4ZV, who also included some reference
links.
I got a chuckle from the opening sentence of one link, from Inrad: "
"roofing filter" is the current buzzword in high-end radios. "
I agree with that, though there is no doubt that a better first IF
filter can make a big improvement in receiver performance.
The way the term is being used in some advertisements sort of reminds me
of "Solid State", "Transistorized", and "Digital". And remember "Color"
TV antennas?
Most everyone agrees that (lately) a roofing filter is the first
bandpass filter after the first mixer in a superheterodyne receiver.
(Once upon a time a roofing filter was a lowpass filter, not necessarily
in the IF of a receiver.) There is some consensus that a roofing filter
is not the narrowest or final filter (or filtering process in the case
of a DSP receiver) in the IF. Perhaps the later, narrower filters are
"ceiling filters." It seems that we have always had roofing filters, we
just didn't call them that until a few years ago.
There is also an implication that a roofing filter must be a crystal
filter with a fairly narrow bandwidth, on the order of a few times the
bandwidth needed for the mode of operation. I have not yet heard anyone
refer to an LC BPF or IF transformer as a "roofing filter". Or perhaps
the first filter after the mixer is always a roofing filter, regardless
of type, or bandwidth?
Is the first IF transformer in my Hammarlund Super-Pro a roofing filter?
DE N6KB
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