开云体育

Hi Hugh,

Actually, all receivers with an up-converting first mixer use one or more
roofing filters which do not determine the receiver's ultimate selectivity.

A roofing filter is defined as an IF filter immediately following a mixer.
The purpose of the roofing filter is twofold:

1. To reject the image resulting from the mixing process.

2. To reduce the statistical likelihood that strong signals falling within
the bandwidth of the RF preselector, but outside the bandwidth of
selectivity-determining IF filters further down the receiver's IF chain,
will overdrive the ADC and/or the IF stages preceding it. (In an analogue
receiver, these signals can overload mixers and IF amplifiers downstream of
the roofing filter, if the roofing filter passes them.)

The roofing filter is not a selectivity-determining filter. It must be
sufficiently wide to pass the entire TOBW of the widest emission the
receiver is intended to process.

In the Icom Pro series, both the 1st and 2nd mixer are followed by roofing
filters. The filter centre frequencies are 64.455 MHz and 455 kHz
respectively, and their -6 dB bandwidth is 15 kHz to pass FM. The IF-DSP
filters determine the receiver's ultimate selectivity.

Perhaps the most authoritative short article on roofing filters I have seen
is one which my friend George W5YR (sadly, SK) contributed to my
IC-756Pro/Pro2 User Review in 2002:



Another fine article on this topic, also authored by George:



Cheers for now, 73,
Adam VA7OJ/AB4OJ