In an SSB/CW receiver, what is the purpose of the mixer?

In a superheterodyne receiver, the mixer is the critical stage where the incoming amplified radio frequency (RF) signal is combined with an unmodulated, internally generated signal from the local oscillator (LO). This non-linear mixing process creates new frequencies, specifically the sum and the difference of the original two frequencies. The receiver then filters these products to extract a constant, lower frequency known as the intermediate frequency (IF). Converting the variable RF signal to a fixed IF allows the rest of the receiver to use highly optimized, fixed-frequency filters and amplifiers, improving performance. The mixer does not generate audio or remove the carrier; that is the job of the product detector located further down the receiver chain.