Frequency shift key (FSK) is a digital modulation scheme using two or more output frequencies. Amplitude shift key (ASK) transmits data by varying the amplitude of the transmitted signal. Frequency modulation (FM) causes the instantaneous frequency of a sine wave carrier to depart from the center frequency by an amount proportional to the instantaneous value of the modulating signal. Amplitude modulation (AM) causes the baseband signal to vary the amplitude or height of the carrier wave to create the desired information content. On-off key (OOK), the simplest form of modulation, consists of turning the signal on or off. Selecting RF receivers requires an understanding of modulation methods such as AM and FM. With frequency hopping spread spectrum, signals move through a narrow set of channels in a sequential, cyclical, and predetermined pattern. With direct sequence spread spectrum, signals are spread over a large band by multiplexing the signal with a code or signature that modulates each bit. Radio techniques limit localized interference and noise. There are several ways to decode or modulate this information, including amplitude modulation (AM) and frequency modulation (FM). Detectors or demodulators then extract information that was encoded before transmission. RF receivers use an antenna to receive transmitted radio signals and a tuner to separate a specific signal from all of the other signals that the antenna receives. RF receivers are electronic devices that separate radio signals from one another and convert specific signals into audio, video, or data formats.
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