A direct-conversion receiver (DCR) is a radio receiver that reproduces the original information signal, which could be sound, video, or data, from the originating source or a radio transmitter, the source of the on-air signal. It does not require shifting of the received signal into an intermediate signal. A direct-conversion receiver has fewer parts than about half the other types of radio receivers. Inside a direct-conversion receiver is a radio source, known as the local oscillator, which is tuned to the same frequency as the on-the-air signal.
The radio reception process begins with extracting the on-the-air signal, also known as an electromagnetic (EM) wave, which was previously sent or transmitted into the air by a radio transmitter. It is then converted into electrical signals at the same frequency in cycles per second as the transmitted signal by the antenna of the radio receiver. For example, a radio signal at 500 kilohertz (kHz), or 500,000 cycles per second, from a transmitter will travel through the air as on-the-air signal, reaches the receiver antenna, and becomes electrical signals at the same frequency, or 500,000 cycles per second, at the receiver input. The levels are usually very low at the receiver input. This requires sensitive circuitry inside the DCR that will amplify the received signal.
An EM wave at the receiver input has already been modified in some form by the transmitter to carry the original signal or information. When the average level of the EM wave is modified, it is called amplitude modulation (AM). When the instantaneous frequency of the EM wave is in proportion to the information source, it is referred to as frequency modulation (FM). These are the two common modulations or methods for including information into an EM wave or a radio frequency (RF) carrier.
Once the RF carrier is at the input of a direct-conversion receiver and the DCR is tuned to it, the modulation can be extracted using a principle known as mixing and detection. In the mixing process, the signal from the transmitter and the signal from a local source, known as local oscillator, inside the DCR are used. The two signals mentioned will be fed to the mixer detector, and the result is the information from the transmitter.
A direct-conversion receiver is also referred to as a homodyne because it uses a single frequency. It is also known as a synchrodyne because it uses a sychronized RF source, or almost the same phase and frequency as the received signal. Other types of receivers require another frequency, or intermediate frequency (IF), to further process the received signal. The DCR needs no IF, which is why it is also called a zero-IF receiver.