The first mixer in the receiver mixes the incoming signal with the local oscillator to produce:

The core principle of a superheterodyne receiver is heterodyning, which means mixing two frequencies together to create new frequencies. The first mixer combines the incoming radio frequency (RF) signal from the antenna with a signal from the local oscillator (LO). The mathematical result of this mixing process is the creation of sum and difference frequencies. The receiver's circuitry is designed to select one of these resulting frequencies to serve as the Intermediate Frequency (IF). This IF is then passed on to the IF amplifiers for further filtering and amplification. It is important not to confuse the first mixer with the detector (which acts as a second mixer in some designs). The first mixer always produces the intermediate frequency, while the detector produces the audio frequency.