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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Variable-frequency oscillator ： ウィキペディア英語版 Variable-frequency oscillator A variable frequency oscillator (VFO) in electronics is an oscillator whose frequency can be tuned (i.e. varied) over some range. It is a necessary component in any tunable radio receiver or transmitter that works by the superheterodyne principle, and controls the frequency to which the apparatus is tuned. ==Purpose== In a simple superhet radio receiver, the incoming radio frequency signal (at frequency $f\_$) from the antenna is ''mixed'' with the VFO output signal tuned to $f\_$, producing an intermediate frequency (''IF'') signal that can be processed downstream to extract the modulated information. The IF signal frequency is chosen to be either the sum of the two frequencies at the mixer inputs (up-conversion), $f\_+f\_$ or more commonly, the difference frequency (down-conversion), $f\_-f\_$, depending on the receiver design. In addition to the desired ''IF'' signal and its unwanted image (the mixing product of opposite sign above), the mixer output will also contain the two original frequencies, $f\_$ and $f\_$ and various harmonic combinations of the input signals. These undesired signals are rejected by the IF filter. If a double balanced mixer is employed, the input signals appearing at the mixer outputs are greatly attenuated, reducing the required complexity of the IF filter. The advantage of using a VFO as a heterodyning oscillator is that only a small portion of the radio receiver (the sections before the mixer such as the preamplifier) need to have a wide bandwidth. The rest of the receiver can be finely tuned to the IF frequency. In a direct-conversion receiver, the VFO is tuned to the same frequency as the incoming radio frequency and $f\_=0$ Hz. Demodulation takes place at baseband using low-pass filters and amplifiers. In a radio frequency (RF) transmitter, VFOs are often used to tune the frequency of the output signal, often indirectly through a heterodyning process similar to that described above.〔 Other uses include chirp generators for radar systems where the VFO is swept rapidly through a range of frequencies, timing signal generation for oscilloscopes and time domain reflectometers, and variable frequency audio generators used in musical instruments and audio test equipment. 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「Variable-frequency oscillator」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.