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Direct-conversion receiver
A direct-conversion receiver (DCR), also known as homodyne, synchrodyne, or zero-IF receiver, is a radio receiver design that demodulates the incoming radio signal using synchronous detection driven by a local oscillator whose frequency is identical to, or very close to the carrier frequency of the intended signal. This is in contrast to the standard superheterodyne receiver where this is accomplished only after an initial conversion to an intermediate frequency.
The simplification of performing only a single frequency conversion reduces the basic circuit complexity but other issues arise, for instance, regarding dynamic range. In its original form it was unsuited to receiving AM and FM signals without implementing an elaborate phase locked loop. Although these and other technical challenges made this technique rather impractical around the time of its invention (1930's), current technology and software radio in particular have revived its use in certain areas including some consumer products.
==Principle of operation==
The direct-conversion receiver feeds the radio frequency signal into a frequency mixer, just as in a superheterodyne receiver. However unlike the superheterodyne, the frequency of the local oscillator is not offset from but right at the received signal's frequency. The result is a demodulated output just as you would obtain from a superheterodyne receiver using synchronous detection (a ''product detector'') following an intermediate frequency (IF) stage. In other words, the conversion to baseband is done in a single frequency conversion. This avoids the complexity of the superheterodyne's two (or more) frequency conversions, IF stage(s), and image rejection issues.
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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