Roofing filter

A roofing filter, is a type of audio 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 1. 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 is not performed 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 usually uses a high 1. IF of higher than 40 MHz. Because of this, roofing filters are usually crystal filter types.