beat frequency

The term beat frequency is often seen regarding sound waves, but refers to a general property of waves also applicable to electromagnetic radiation (EMR) and to oscillations within electronic circuits. Beat frequencies are a consequence of interference between waves; if waves of different frequency are superimposed, a type of oscillation with a frequency that is the difference of the two results. The term heterodyne (literally meaning more than one frequency) is used to indicate a device that makes use of this type of interference. The phenomenon applies to oscillating electronic signals such as those stemming from waves (e.g., via an antenna), and it is commonly used as a method within electronics to lower the frequencies of signals; there is a regime of frequencies most easily handled by electronic circuits and shifting frequencies toward that regime is a common way to deal with very high frequencies. Practical electronics circuits dealing with radio waves and their high frequencies typically make use of this technique, such as everyday consumer radios, typically described as superheterodyne. The general method is that the circuit include an oscillator (local oscillator, abbreviated LO) that produces a chosen fixed frequency, which is combined with a received signal. The resulting difference-frequency, which is lower than the frequency received, retains the variations in amplitude and frequency, but within a frequency range more practical for electronic manipulation and analysis.

Radio telescope receivers commonly use the technique (typically indicated by the term heterodyne) combining the electronic signal received from the antenna with that of its local oscillator, so that subsequent processing can be carried out on a corresponding signal with a lower frequency. For shorter wavelengths (and higher frequencies, such as millimeter-to-far infrared range, submillimeter and terahertz), there comes a point when effective antennas, electronic transmission and circuitry for the signal's frequency are impractical or impossible, and some devices for this frequency-range use an analogous technique within their optics: an EMR source such as a laser serves as a local oscillator, its waves mixed with the received EMR, producing EMR at a lower frequency which can be detected and analyzed using techniques of receivers for that lower frequency-regime. The term heterodyne photodetection indicates this, and if the result is analyzed for its spectrum in further electronics, a term for the entire device is heterodyne spectrometer.