The mixer stage of a superheterodyne receiver:

The hallmark of a superheterodyne receiver is its ability to convert varying incoming RF signals to a constant, lower frequency for easier filtering and amplification. The mixer stage is the component responsible for this frequency conversion. By heterodyning (mixing) the incoming RF signal with a local oscillator signal, the mixer produces sum and difference frequencies. The receiver's tuned circuits then select the difference frequency to serve as the intermediate frequency (IF). While mixers can inadvertently produce spurious signals (intermodulation products) if overdriven, producing the IF is their primary, intended function. Demodulating SSB signals is handled later by a product detector, and buffer stages simply isolate circuits without altering their frequency.