I came along when many hams still used TRF and regeneratve receivers, well
before up conversion and "roofing filter," or any sort of filter other than the
crystal phasing filters, low frequency IF's, and audio filters of the
"Selectoject" variety were unheard of. Back then, you put your IF just below
the bands you wanted to cover; i.e. 455 kHz for a typical 550 kHz, kc, to 32
mHz, Mc, communications reciever.
And the 455 (or 456) kHz IF was often followed by a 262, 85, 50, or 25 kHz
second IF. Or a combination, such as an 85 kHz IF for AM phone, followed a
third dedicated 25 kHz IF for CW. Plus a phasing crystal filter in the first or
second IF, and audio peaking/notching of course. All of which gave pretty good
selectivity, especially for the Korean War era.
Of course, "images," for example you want to talk to a station on 3.900 so your
Local Oscillator is on 4355 - and there's a strong commercial station 455 Kc
above your LO at 4810 - were a problem, even then. You got to listen to your
buddy and whatever was going on at the commercial station. Not good for Amateur
use, and a disaster in the making for military and commercial services. So
reciever designers shifted the IF frequency up to the lightly used segments of
the MW and "Tropical Band," and added extra stages of tuned RF to improve image
rejection.
At that time, most stations above 1500 kHz were 250 watts, while Loran kicked
in with some serious emitters just above 1.750. So the unused segment at 1650
kHz became a common IF frequency, and "Band 1" on many SW recievers was 550 to
1600 kHz, while "Band 2" was 1750 to 5250 mHz (or so) with a substantial gap at
the IF frequency.
Since quality communications recievers of the era had at least one, and often
two or three tuned RF circuits before the mixer, images were not a serious or a
frequent problem. Of course, the Tech who had to make all those bandswitched
tuned circuits connected to the four and five and six gang "bread slicer"
variables track with each other and the first LO had a bear by the tail but
that's another story.
Then came the revolution! SIMPLE IS BETTER. Sometimes.
When the built in preselection provided by tuned grid, tuned plate RF amps,
often followed by a tuned grid mixer, gave way to the bandpass filters or low
pass filters in "modern high performance recievers" images became a problem
again. And the "gap in coverage" probably didn't suit the sales department.
Continuous coverage sounds so much better!
So we got up conversion. Sometimes way up, well into the VHF range. Up where
comparatively narrow filters are either expensive or unobtainable. So 15 kHz
filters with a swept passband than looks like rather like a first grader's
drawing of half an orange on a plate became "the standard." With the result you
know.
We got roofing filters, so called by anology. A roof lets precipitation fall
everywhere except the area covered by the roof. A roofing filter "sheds" all
the "stuff" outside the desired pass band and lets the signals in the pass band
enter the IF strip. Which should be good enough for less than critical
applications.
In principle an IF at 80, or 180, mHz could be built with selectivity equal to
any other IF. But you can't get suitable filters for those frequency ranges.
And until we get a major and unlooked for breakthrough in filter technology up
conversion recievers will not have the selectivity we need to cope with our
increasingly crowded bands. So we live with high performance recievers that are
not continuous coverage - or with lower performance recievers that do provide
continuous coverage.
Merry Christmas to all
Pete Allen AC5E
> Merry Christmas to the reflector
>
> Please explain the term "roofing filter". What type of filter is it, and how
> does it get its name? How does it differ from the standard filters with
> which I am familiar?
> Seasons greetings to all and thanks in advance.
>
> Ron
> K3MIY
>
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