A roofing filter is a type of filter use 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 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.