Roofing filter

A roofing filter is a type of filter used in a HF receiver (shortwave radio). It is usually found after the first receiver mixer. The goal of a roofing filter is to reduce the passband of the 1st intermediate frequency (IF) to about 6-20 kHz, so that overloading and distortions by the following amplifier stages and mixers are reduced. The receiver's Bandwidth (signal processing) is not determined by the roofing filter but by a following crystal filter, mechanical filter or DSP filter. These allow much better filtering curve than a roofing filter, which often uses a high 1st IF of higher than 40 MHz. Because of this, roofing filters are usually crystal filter types.

It should be noted that while a 6-20 kHz roofing filter is acceptable for general purpose HF radio reception, demanding uses like listening to weak CW or SSB signals requires the use of roofing filters that are a much smaller width appropriate to the reception mode in use. 250 Hz, 500 Hz, or 1.8 kHz would be acceptable values. These also require that the receiver uses a low first IF below VHF range, perhaps 9 or 11 MHz.