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Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden, in which new frequencies are created by combining or mixing two frequencies. Heterodyning is used to shift one frequency range into another, new one, and is also involved in the processes of modulation and demodulation.〔 The two frequencies are combined in a nonlinear signal-processing device such as a vacuum tube, transistor, or diode, usually called a ''mixer''.〔 In the most common application, two signals at frequencies ''f1'' and ''f2'' are mixed, creating two new signals, one at the sum ''f1'' + ''f2'' of the two frequencies, and the other at the difference ''f1'' − ''f2''.〔 These new frequencies are called heterodynes. Typically only one of the new frequencies is desired, and the other signal is filtered out of the output of the mixer. Heterodynes are related to the phenomenon of "beats" in acoustics. A major application of the heterodyne process is in the superheterodyne radio receiver circuit, which is used in virtually all modern radio receivers. ==History== In 1901, Reginald Fessenden demonstrated a direct-conversion heterodyne receiver or beat receiver as a method of making continuous wave radiotelegraphy signals audible. Fessenden's receiver did not see much application because of its local oscillator's stability problem. While complex isochronous electromechanical oscillators existed, a stable yet inexpensive local oscillator would not be available until Lee de Forest's invention of the triode vacuum tube oscillator.〔, stating "Fessenden's circuit was ahead of its time, however, as there simply was no technology available then with which to build the required local oscillator with the necessary frequency stability." Figure 7.10 shows a simplified 1907 heterodyne detector.〕 In a 1905 patent, Fessenden stated the frequency stability of his local oscillator was one part per thousand. Early spark gap radio transmitters sent information exclusively by means of radio telegraphy. In radio telegraphy, the characters of text messages are translated into the short duration dots and long duration dashes of Morse code that are broadcast as bursts of radio waves. The heterodyne detector was not needed to hear the signals produced by these spark gap transmitters. The transmitted damped wave signals were amplitude modulated at an audio frequency by the spark. A simple detector produced an audible buzzing sound in the radiotelegraph operator's headphones that could be transcribed back into alpha-numeric characters. With the advent of the arc converter, continuous wave (CW) transmitters were adopted. CW Morse code signals are not amplitude modulated, so a different detector was needed. The direct-conversion detector was invented to make continuous wave radio-frequency signals audible. The "heterodyne" or "beat" receiver has a local beat frequency oscillator (BFO) that produces a radio signal adjusted to be close in frequency to the incoming signal being received. When the two signals are mixed, a "beat" frequency equal to the difference between the two frequencies is created. By adjusting the local oscillator frequency correctly, the beat frequency is in the audio range, and can be heard as a tone in the receiver's earphones whenever the transmitter signal is present. Thus the Morse code "dots" and "dashes" are audible as beeping sounds. This technique is still used in radio telegraphy, the local oscillator now being called the beat frequency oscillator or BFO. Fessenden coined the word ''heterodyne'' from the Greek roots ''hetero-'' "different", and ''dyn-'' "power" (cf. δύναμις or dunamis).〔(Tapan K. Sarkar, History of wireless, page 372 )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heterodyne」の詳細全文を読む スポンサード リンク
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