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What Is a Local Oscillator?

Geisha A. Legazpi
Geisha A. Legazpi

A local oscillator is a device that generates a sinusoidal signal with a frequency such that the receiver is able to generate the correct resulting frequency, or intermediate frequency (IF), for further amplification and conversion into audio detection. There is one local oscillator in a single conversion superheterodyne receiver where heterodyning or mixing is used to generate beat frequencies, which may be the sum or the difference of two frequencies. The local oscillator is usually adjustable and in step with the increment or decrement in the receiver frequency. For instance, if the receiver is tuned to 1,455 kilohertz (kHz) as radio frequency input (RF-in), the local oscillator frequency (LOF) may be set to 1,910 kHz for a so-called high side injection. The two signals are fed to an electronic device known as the mixer, which derives LOF - RF-in = IF or 455 kHz, which suggests why amplitude modulation (AM) broadcast receivers have about four stages of low-power amplifiers tunable to 455 kHz.

The most common way to add message into a radio frequency carrier is a process called AM, where the average peak amplitude of the carrier is made proportional to the message. When a radio frequency carrier has been generated, the secondary winding of an audio transformer in series with the carrier current produces AM when message or modulation is fed to the primary winding. A direct conversion receiver uses a local oscillator tuned to the incoming radio frequency. Using homodyne detection, the LOF and RF-in are mixed, producing a low-pass filtered output, which is the message in AM. On the other hand, there are receiver designs that require dual conversion and use two local oscillators and two intermediate frequencies.

Woman holding a disc
Woman holding a disc

Frequency modulation (FM) broadcast receivers may use phase-locked loop detection for converting FM back into audio. The message is proportional to the instantaneous deviation of the FM signal from the carrier rest frequency, thus a phase-locked loop that maintains lock on the FM signal will generate a steering voltage with an alternating current (AC) component proportional to the message; this is phase-locked loop detection. In heterodyne detection, the local oscillator may be tuned to a slightly different frequency such as 1 kHz higher or lower than the incoming radio frequency. The result is a 1 kHz audio at the detector output, which could drive a headset or a speaker for the decoding of Morse code, a conversion of individual letters into a series of short and long bursts of signals.

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