downconverter

A downconverter is an electronic device that converts a higher-frequency radio signal to a lower-frequency signal by mixing it with a local oscillator signal. The result is typically a lower, intermediate frequency (IF) that can be more easily processed by subsequent circuitry.

In operation, a mixer combines the incoming RF signal with the local oscillator (LO). The mixer outputs several frequencies, including the sum and difference of the two frequencies. The system then filters to select the difference frequency, which is the IF. The relationship is commonly expressed as f_IF = |f_RF − f_LO|. Engineers may use high-side or low-side injection, depending on the preferred LO frequency and to manage image frequencies and filter requirements.

Downconverters come in several forms. Passive downconverters use diodes or passive mixer networks, while active downconverters employ transistor-based mixers to provide conversion gain. Some devices are integrated into one package, especially in satellite receivers or software-defined radios. A common practical example is the satellite dish low-noise block downconverter (LNB), which converts signals from around 11–12 GHz down to approximately 950–2150 MHz for processing by a terrestrial receiver.

Applications include superheterodyne receivers in radio, television, and satellite systems, as well as test equipment and some software-defined radio architectures. Key design considerations include conversion gain or noise figure, LO leakage, image rejection, isolation between ports, and the quality of the IF filtering that follows the mixer.