Heterodyne

Heterodyne (from the Greek. Ἕτερος - other; δύναμις - force) is a low-power generator of electrical oscillations used to convert the signal frequencies in superheterodyne radios , direct conversion receivers , wavemeters , etc.

Initially, the local oscillator was called a radio receiver. , in which there was an additional high-frequency generator tuned to a frequency close to the frequency of the received signal , which increased the sensitivity of the radio receiver. Later, after the invention of the super-heterodyne, this generator began to be called the local oscillator.

The local oscillator creates auxiliary frequency oscillations, which in the mixer unit are mixed with high frequency oscillations coming from outside. As a result of mixing the two frequencies, the input and the local oscillator, two more frequencies are formed (total and differential). The difference frequency is used as an intermediate frequency at which the main signal amplification occurs.

To the local oscillators set high requirements for the stability of frequency and amplitude , as well as the spectral purity of harmonic oscillations . The higher these requirements, the more difficult the design of the local oscillator: stabilize the supply voltage, apply complex circuits that exclude the influence of external factors on the generator frequency, components with special properties, the local oscillator is placed in a thermostat , use automatic frequency control systems, etc. If the local oscillator works at a fixed frequency, stabilization is applied using a quartz resonator . In modern radio equipment, digital frequency synthesizers , which have several important advantages, are increasingly used as tunable local oscillators.