The mixer or mixing stage is an electronic circuit that mixes two frequencies, usually an input frequency and an oscillator frequency, and generates various frequencies from them. During mixing, which can only be performed on active electronic components with nonlinear characteristics, the sum (fo + fe) and difference (fo - fe) frequencies of the oscillator and input frequencies are produced in addition to the input frequency (fe) and the oscillator frequency (fo).
In the spectral view, the sum and difference signals lie at the same frequency distance from the oscillator frequency, one above, the other below. These frequencies are therefore also called mirror frequencies. In addition to these frequencies, there are also various mixing frequencies, which are formed from the harmonics of the oscillator frequency with the input frequency.
The purpose of a mixer is to convert a high frequency RF input frequency to a lower frequency RF signal by mixing it with an oscillator frequency. Since different frequencies are generated during mixing, the frequency is filtered out and selectively amplified in the following amplifier stages, the IF amplifier. The mixer principle has been used in all superheterodyne receivers since its invention in 1925.
The example of an FM receiver is used to illustrate the mixer principle. The FM modulated input frequencies are between 87.6 MHz and 108 MHz. If the input frequency is mixed with an adjustable oscillator frequency from 98.3 MHz to 118.7 MHz, the result is a difference frequency of 10.7 MHz, both at the lowest input frequency (98.3 MHz - 87.6 MHz) and at the highest input frequency (118.7 MHz - 108 MHz). This difference frequency is called the intermediate frequency. It contains the modulated audio signal and is amplified in the selectively operating intermediate frequency amplifier.
A distinction is made between additive and multiplicative mixing. Both methods, which differ in the circuit-technical control of the active components such as electron tubes, transistors, field-effect transistors( FET), etc., have advantages and disadvantages with regard to circuit complexity, the number of mixing products and controllability.