A mixer is a circuit that outputs a signal frequency equal to the sum, difference, or other combination of the frequencies of two input signals. Mixers are typically composed of nonlinear components and frequency selective circuits. The mixer is located after the low noise amplifier (LNA) and directly processes the RF signal amplified by the LNA. To achieve the mixing function, the mixer also needs to receive the local oscillator (LO) signal from the voltage controlled oscillator, and its circuit operates entirely in the RF frequency band.

Mixing refers to the process of transforming a signal from one frequency to another, essentially a process of linear spectral shift. In superheterodyne receivers, the purpose of frequency mixing is to ensure that the receiver obtains high sensitivity, sufficient amplification, and appropriate passband, while also being able to operate stably. The mixing circuit consists of three components: local oscillator, nonlinear device, and bandpass filter. Due to the action of nonlinear components such as diodes, transistors, field-effect transistors, etc., many combined frequency components are generated during the mixing process: p f L ± qf S.

Generally speaking, only f I=f L - f S or f I=f S - f L satisfy the requirements. The method of generating intermediate frequency in the former is called high difference mixing, while the latter is called low difference mixing. Here, a series of combined frequency components generated during the mixing process can be selected to output the corresponding intermediate frequency through a bandpass filter, while other frequency components will be suppressed.