[TenTec] filter

[Ken Brown]

> What is a roofing filter and its advantages. thanks jjdavis
> _
A roofing filter is a low pass filter used in the baseband of an analog 
multiplexed FM microwave radio system. The roofing filter in these 
systems established the high frequency cutoff above which no analog SSB 
multiplex channels could operate.  Noise from the FM discriminator, 
above the roofing filter cutoff frequency, would not be allowed into the 
baseband, and would not be modulated back on to the next FM transmitter 
in a multiple hop microwave system. Since the roofing filter is a low 
pass filter, establishing a top frequency limit, it makes sense to call 
it a roofing filter.

Lately amateurs have been incorrectly calling an IF bandpass filter a 
roofing filter. These filters, being bandpass filters, have a high 
frequency cutoff and a low frequency cutoff, so if they have a roof, 
they also have a floor. They can be of great advantage in a receiver, 
limiting the amount of IF signal power at unwanted frequencies that 
subsequent IF stages have to process. The use of the terminology 
"roofing filter" is an unfortunate misunderstanding of the correct use 
of that term, which persists.

DE N6KB