It's an interesting question when the term "roofing filter" entered
amateur radio circles.
My Drake R7 has the classic up-conversion design with a 48 MHz IF with a
crystal filter immediately following the first mixer, but it is not
described as a "roofing" filter. Here's how Drake describes its function
in the 1979 Service Manual:
...This stage [the first mixer] followed by a four-pole monolithic
48.05 MHz crystal filter. The purpose of this filter is to attenuate
signals removed more than +/- 4 KHz from 48.05 MHz, thus protecting
the remaining stages of the receiver from strong interfering
signals. In this manner, optimum receiver dynamic range is preserved
while providing excellent sensitivity.
Just checked Racal's RA6790/GM manual (1985), which employs a 40.455 MHz
first IF / 20 KHz bandwidth crystal filter and I don't see the word
"roofing filter" used. Rather, it's called simply "bandpass filter" in
both the block diagram and accompanying text.
Jack K8ZOA
Geoffrey Mackenzie-Kennedy wrote:
Just cannot resist saying this Jack, the term "roofing filter" has
certainly been around for a long time - close to 50 years I believe,
possibly longer. I think that I first ran across the term being used
to identify the first IF filter in an Independent Sideband Receiver
which the company for whom I worked manufactured for HF Point-to-Point
applications in the very late 1950's, along with companion high power
ISB transmitters.
Perhaps some other term might be less confusing and prevent any misuse
of the term , but don't let us change from "roofing filter" now!!
73,
Geoff
GM4ESD
Jack Smith wrote:
The term "roofing filter" has been around a long time, at least 20
years.
Usually used in the context of up-converting receivers, where the
first IF is 40.455 MHz, or 45.000 MHz or even higher. The term means
the first selective filter in the receiver. If the first IF is > 30
MHz, the roofing filter is generally wider than one normally needs
for SSB or even AM, and the receiver's ultimate selectivity is
provided in a later IF stage.
As to why it is called a "roofing" filter, I imagine it derives from
the fact that it puts a "roof" over the receiver's selectivity.
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