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Detector (radio)
In electronics, a detector is an older term for an electronic component in a radio receiver that recovers information contained in a modulated radio wave. The term dates from the first three decades of radio (1886-1916). Unlike modern radio stations which transmit sound (an audio signal) on the radio carrier wave, the first radio transmitters transmitted information by wireless telegraphy, using different length pulses of radio waves to spell out text messages in Morse code. So radio receiving equipment didn't have to extract a sound wave from the incoming radio signal, but only "detect" the presence or absence of the radio signal, to produce clicks in the receiver's earphones representing the Morse code symbols. The device that did this was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, and magnetic detector, were used during the wireless telegraphy era.
After sound (AM) transmission took over around 1920, the term evolved to mean a demodulator, a nonlinear rectifier (usually a crystal diode or a vacuum tube) which extracted the audio signal from the radio frequency carrier wave. This is its current meaning, although modern detectors usually consist of semiconductor diodes, transistors, or integrated circuits.
In a superheterodyne receiver the term is also sometimes used to refer to the mixer, the tube or transistor which converts the incoming radio frequency signal to the intermediate frequency. The mixer is called the first detector, while the demodulator that extracts the audio signal from the intermediate frequency is called the second detector.
== Amplitude modulation detectors ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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