Direct-conversion receiver

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The Direct conversion receiver, (DCR) also known as Homodyne, Synchrodyne, or zero-IF receiver, is a radio receiver design with a unique system of demodulating amplitude modulated (AM) signals. It uses a local oscillator, synchronized in frequency to the carrier of the wanted signal, to modulate the received signal.

The wanted demodulated signal is thus obtained immediately as an audio output without further detection, and unwanted signals are left on carriers of the frequency difference between their original carrier and that of the wanted signal. There is no detection of the unwanted signals since the whole signal path is kept free of non-linearity. Unwanted signals can be completely rejected by a low-pass filter in the audio output. The receiver consequently has the advantage of high selectivity, and is inherently a precision demodulator. The principles can be extended to permit separation of signals whose sidebands overlap, and they also lead to improved detection of pulse-modulated signals.

The Homodyne was developed in 1932 by a team of British scientists searching for a method to surpass the superheterodyne. This new type receiver later renamed the Synchrodyne proved to have superior performance and the single conversion stage meant that the direct conversion receiver had lower complexity and power consumption. But the circuit after a period of time became unstable due to slight drift in frequency of the local oscillator. To counteract this drift, the frequency of the local oscillator was compared with the input by a phase detector so that a correction voltage would be generated and fed back to the local oscillator, thus keeping it on lock. This type of feedback circuit evolved into what is now called a Phase-locked loop. While the method has existed for several decades, it has been difficult to implement due largely to component tolerances, which must be very high for this type of circuit to operate well.

The circuit was not without other problems. Reverse transmission paths can occur in the receiver. Local oscillator energy can leak through the mixer to the antenna input and then re-enter the mixer. The overall effect is that the local oscillator energy would self-mix and create a DC offset. The offset could be large enough to swamp the baseband amplifiers and destroy signal reception. There were several work arounds to deal with this issue but these too added to the complexity of the receiver. Ultimately the higher costs were found to outweight the benefits.

The widespread use of this principle did not begin until the development of the integrated circuit and incorporation of complete phased-lock loop devices in low-cost IC packages. These are no longer limited to the reception of AM radio, now being able to process more complex modulation schemes. Since then, this has become the most widespread method of receiving radio signals and it is incorporated in everything from cellphones and televisions to avionics and medical imaging apparatus.

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