Local Oscillator: Unveiling the Core of Radio Frequency Signal Processing

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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.

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.