[Elecraft] [K3] Roofing filters are misunderstood

[Peter Torry]

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
> dezrat at outlook.com
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