In a superheterodyne receiver, which stage allows detection to function at a single frequency regardless of the received frequency?

The defining characteristic of a superheterodyne receiver is its ability to convert any incoming radio frequency into a single, fixed internal frequency known as the Intermediate Frequency (IF). This clever conversion is performed by the Mixer stage. The Mixer combines the incoming RF signal with a locally generated frequency (from the Local Oscillator). Through a process called heterodyning, it outputs the sum and difference of these two frequencies. By tuning the local oscillator, the difference frequency is always forced to be the fixed IF. Because the Mixer constantly outputs this single IF, all subsequent stages (like the IF filter, discriminator, or detector) only ever need to operate at one specific frequency. This allows those later stages to be highly optimized and rigidly filtered, improving receiver performance.