Here are a couple of good explanations of roofing filters from the Elecraft mailing list:
- Don, W3PFR, writes:
The ultimate selectivity of the K3 is determined by the DSP.
Now think about the input to the DSP – if one restricts the number of signals entering the DSP (actually the ADC front end for the DSP), the DSP will not have to deal with as much ‘garbage’. The more narrow the roofing filter, the less ‘garbage’ will be present at the DSP front end.
Consider also that the DSP can handle an S-9 +20 signal input from an unwanted signal – above that level it will be overloaded. In situations where signals greater than that level are encountered within the passband of the roofing filter, the Hardware AGC will limit the signal level presented to the DSP. Even though the DSP may be cranked down to hear only the desired signal, the signal gets through to the DSP front end and will reduce the gain of the receiver because the Hardware AGC is being activated by the strongest signal within the passband of the roofing filter – the result is AGC “pumping” as a result of a signal that one may not even hear. Using a more narrow roofing filter can eliminate this unwanted signal and eliminate the AGC pumping.
Think of the roofing filter as a roof on a house – the wide filter allows the DSP (and hardware AGC) to respond to all the rain falling on the roof. To reduce the amount of rain falling on the roof, a more narrow roofing filter is required.
If you are interested in contesting and/or heavy DXing, you are liable to encounter nearby unwanted strong signals within the passband of the roofing filter which will cause the hardware AGC ‘pumping’, and you should invest in narrow filters that will eliminate that effect. For those whose operation is more casual and do not wish to dig for weak signals in the presence of stronger nearby signals, a wide roofing filter will do the job nicely.
- What “Roofing Filter” means to Elecraft. This is an explanation on the Elecraft website.