In order for us to have an intelligent discussion about roofing filters,
we need to agree on a definition of roofing filter. I don't recall
hearing that term prior to a few years ago. By the usage I have seen
here on this email reflector it seems that "roofing filter" refers to a
filter in the first IF of a receiver. It is unclear to me whether that
would mean that the two pole,15 kHz BW monolithic crystal filter in my
Omni VI is the roofing filter or eight pole 2.4 kHz BW filter is the
roofing filter. Or perhaps that term does not even apply to the Omni VI
without modification. In order for a filter to be a "roofing filter"
does it have to follow the first mixer (be in the first IF) and precede
any gain stages in the first IF? Does it have to be fairly narrow, like
on the order of a couple times the bandwidth of the mode beiing used, or
less? Or can it be any filter in the first IF? Certainly bandpass
filters in the RF stages before the first mixer do not qualify as
roofing filters, yet they do perform a similar function, in that they
limit the amount of total power that the following stages have to deal
with, without producing IMD. What IS a roofing filter?
DE N6KB
I guess we don't have a complete meeting of the minds.
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