[Elecraft] here is how to "get a feel" for a roofing filter

[Eric Swartz - WA6HHQ, Elecraft]

The confusion here may be primarily one of semantics. The K3's main 
narrow IF filters -are- the first filters seen after the first mixer, 
just like those in the K2. Our first IF at 8.215 MHz acts as our primary 
narrow IF filter stage and can also be considered our roofing filter 
stage, since it is followed by the DSP IF and its additional filtering. 
We had so many questions about what our 'roofing' filters were on the K2 
and K3,  that we decided to refer to the 8.215 MHz narrow filters as 
roofing filters. Since they are the first IF filter this is also 
technically correct.  :-)

In our case, since the roofing filters ARE the main, narrow, IF filters 
(just like the 4.915 1st IF filter is in the K2),  dynamic range is 
measured for signals OUTSIDE the filter b/w, not inside. The K3's 1st IF 
(8.215 MHz) can easily use a crystal filter down to 200 Hz. Higher 
frequency first IF stages in other radios are limited to about 3 kHz 
minimum and typically use filters in the 5 kHz to 10 kHz width range. 
They only have one filter at their 1st IF (their roofing filter) so it 
has to be at least wide enough to handle SSB. Plus it is very difficult 
to make a filter narrower than 3 kHz economically at an IF of 40-70 MHz.

On traditional up-converting IF radios with wide (>=3 kHz) roofing 
filters at their first IF (40 MHz to 70 MHz) the interfering signal can 
easily be inside their first roofing filter but outside their next stage 
narrow (500 Hz etc) IF filters. This causes havoc with the stages 
between their roofing filter and their narrower IF filters.  (The ARRL 
test sweeps to 1 kHz away with a 500 Hz main IF filter.)

73, Eric   WA6HHQ
Elecraft