Local oscillator
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A local oscillator is a device used to generate a signal which is beat against the signal of interest to mix it to a different frequency. The oscillator produces a signal which is injected into the mixer along with the signal from the antenna in order to effectively change the antenna signal by heterodyning with it to produce the sum and difference (with the utilization of trigonometric angle sum and difference identities) of that signal one of which will be at the intermediate frequency which can be handled by the IF amplifier. These are the beat frequencies. Normally the beat frequency is associated with the lower sideband, the difference between the two.
Several local oscillators can be strung in series to form a local oscillator chain (LO chain).
The most of principles listed below, were widely used in radio physics, for frequencies from kHz to MHz. After spreading of nonlinear optics, the same terms and principles apply also to the optical frequiencies of order ot 1015Hz.
