Direct Conversion Radio Receiver

 We have discussed tuned radio frequency receivers and superheterodyne receivers. Among superheterodyne receivers we have seen both upconverting superhets and the conventional downconverting superhets. Another type of radio receiver is the direct conversion radio receiver, which though invented in 1930s, was impractical with the technology available then. With advent of modern integrated circuits and software defined radios, it became possible to implement direct conversion receivers, which find application in mobile phones, televisions and medical imaging as well as in software defined radios. Direct conversion receivers are also known as homodyne, synchrodyne or zero-IF receivers. They demodulate the incoming radio signal by using synchronous detection and hence the name synchrodyne.

Direct conversion receivers use a local oscillator frequency which is identical to or very close to the carrier frequency of the received signal so that no intermediate frequency or IF is generated. The output of the mixer stage is a baseband signal, usually the audio signal modulating the carrier in the received signal. The sum of received signal and local oscillator frequencies also appear at the output of the mixer. But they are easily rejected by a low pass filter. Single conversion reduces the complexity of the circuit, though with some disadvantages. As there is no high gain IF amplifier with automatic gain control, the output level of the frequency mixer may vary widely depending on the received signal strength.

Moreover, direct demodulation of the signals requires phase locking the local oscillator to the carrier frequency, which is technically demanding. Instead of the usual envelope detector used in superhets, direct conversion receivers use quadrature detection followed by digital signal processing. Two quadrature outputs can be processed using software defined radio techniques to perform any sort of demodulation. Quadrature outputs are at a phase difference of 90 degrees, which correspond to a quarter of the full 360 degree phase of a wave and hence the name. These two outputs are called in-phase and quadrature (I/Q) signals. I/Q signals are produced by sending local oscillator signals to two mixers with each input differing by 90 degrees in phase.