Hi Bob,
Roofing filter is a term I have yet to learn.
Your thoughts would be appreciated.
Once upon a time in the days of analog multiplexed microwave
systems, multiple voice grade audio channels were converted to SSB
signals and combined together and called the baseband, to modulate a
wideband microwave FM transmitter. At the receiving end the output from
the FM discriminator was the receive baseband, and was fed to a bunch of
SSB receivers, to demodulate back down to voice grade audio. Actually
each "multiplex channel modem" included both transmit and receive
functions, along with E & M signaling which was essentially the on hook
/ off hook indicator for a telephone circuit. The output of the FM
discriminator in the receiver included all of the multiplex channel
slots, which were normally spaced by 4 kHz (and had about 3 kHz voice
grade audio bandwidth), from about 15 kHz up to a few megahertz,
depending on the channel capacity of the system. Above the highest
channel frequency slot there was HF noise from the discriminator, which
was filtered out with a "roofing filter". The baseband coulld be sent to
the next microwave transmitter in the system, for the next hop from
mountain top to mountain top, without fully demodulating back to audio
channels. You did not want to modulate the transmitter with any of that
"higher than the highest multiplex channel noise" so you used the
roofing filter to limit the frequency response of the baseband
modulating the transmitter. The Roofing Filter was a low pass filter,
not a band pass filter, and the high frequency cutoff was just above the
top multiplex channel frequency slot. That top frequency would be called
the "roof" of the system, all usable channels were between 0 Hertz and
the top end "roofing" frequency. (actually the bottom ten or fifteen
kilohertz was reserved for the "orderwire" and alarm signalling
systems). So this filter defined the top of the baseband frequency
response, and was called a "roofing filter"
Then a few years ago communications radio manufacturers started
putting narrower bandpass filters earlier in the IF signal chains of
mostly HF radios, and started calling them roofing filters. In this
application they don't just determine a high frequency cutoff, they also
determine a low frequency cutoff, as they are bandpass filters and not
lowpass filters, so the use of the term "roofing filter" does not
exactly conform to it's former use in analog multiplex systems. There
are a lot of advantages to having them, even if the name they go by is
somewhat incorrect.
DE N6KB
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