Bill,
Originally the superhet receivers converted from the signal frequency to
a lower fixed intermediate frequency in order to obtain sufficient
selectivity. IFs went as low as 85kHz until crystal filters became more
widespread. As designs changed ( changed not improved) it became usual
to convert to a higher frequency, in the order of 70MHz to obtain good
image rejection because manufacturers economised on signal frequency
filtering and they then down converted to a low IF as before. To
protect the second mixer a roofing filter was added and so termed
because it was at the high IF ie the roof in terms of frequency. There
were good reasons for this in commercial equipment and the amateur
equipment followed as a marketing ploy. Hope that helps and please
excuse my poor writing.
Regards
Peter
G3SMT
I still don't get it. What does the word "roof" have to do with
bandpass? That's where the confusion comes from.
73, Bill W6WRT
[email protected]
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