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.

The conversion of the modulated signal 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. The received radio frequency signal is fed directly 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 identical to, the received signal's frequency. The result is a demodulated output just as would be obtained from a superheterodyne receiver using synchronous detection (a product detector) following an intermediate frequency (IF) stage.

To match the performance of the superheterodyne receiver, a number of the functions normally addressed by the IF stage must be accomplished at baseband. Since there is no high gain IF amplifier utilizing automatic gain control (AGC), the baseband output level may vary over a very wide range dependent on the received signal strength. This is one major technical challenge which limited the practicability of the design. Another issue is the inability of this design to implement envelope detection of AM signals. Thus direct demodulation of AM or FM signals (as used in broadcasting) requires phase locking the local oscillator to the carrier frequency, a much more demanding task compared to the more robust envelope detector or ratio detector at the output of an IF stage in a superheterodyne design. However this can be avoided in the case of a direct-conversion design using quadrature detection followed by digital signal processing. Using software radio techniques, the two quadrature outputs can be processed in order to perform any sort of demodulation and filtering on down-converted signals from frequencies close to the local oscillator frequency. The proliferation of digital hardware, along with refinements in the analog components involved in the frequency conversion to baseband, has thus made this simpler topology practical in many applications.

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