Direct-conversion receiver

Direct conversion receivers,(DCR) also known also known as Synchrodyne, Homodyne or zero-IF receivers, are an alternative radio receiver architecture to the well-established superheterodyne, particularly for highly integrated, low-power, low cost units. The fundamental advantage of DCR is that the received signal is amplified and filtered at baseband rather than at a high intermediate frequency. This results in lower current drain in the amplifiers and active filters and eliminates the task of image rejection.

Direct conversion receivers provide a simple means of down-conversion. RF signals are translated down to baseband frequencies by a single conversion stage and a single local oscillator that is at the same frequency of the transmitting station. The single conversion stage means that the direct conversion receiver has lower complexity and power consumption. The method has existed for several decades, but was difficult to implement due largely to component tolerances, which must be very high in this type of circuit.

The method is not without 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 will self-mix and create a DC offset. The offset can be large enough to swamp the baseband amplifiers and destroy signal reception. There are several work arounds to deal with this issue but these add to the complexity of the reciver.