How does a narrow-band roofing filter affect receiver performance?

A roofing filter is a specialized filter placed very early in a receiver's intermediate frequency (IF) chain, typically right after the first mixer. Its primary job is to act as a "roof" protecting the subsequent, more sensitive IF amplifiers and DSP stages from strong out-of-band signals. By using a narrow-band roofing filter, strong adjacent signals are sharply attenuated before they can travel deeper into the receiver. This prevents those nearby strong signals from causing intermodulation distortion or desensitizing the receiver, greatly improving a specification known as "blocking dynamic range." Roofing filters do not lower front-end noise to improve sensitivity. Likewise, they do not inherently improve intelligibility by reducing ringing, which is a problem usually associated with steep audio or late-stage DSP filters.