Communications receivers for the high-frequency (HF) band from 3-30 MHz are asked to perform under difficult conditions, extracting weak signals on frequencies close to the frequencies of unwanted strong signals. These receivers use analog circuitry in the signal path, with basic designs descended from Armstrong's original superheterodyne design. Over the past few years, monolithic analog-to-digital converters (ADCs) and digital downconverters (DDCs) have appeared on the commercial market. These devices permit the construction of inexpensive HF receivers using a signal path that is almost entirely digital.
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