Software-defined radio (SDR) is a radio communication system where components that conventionally have been implemented in analog hardware are instead implemented by means of software on a computer or embedded system. While the concept of SDR is not new, the rapidly evolving capabilities of digital electronics render practical many processes which were once only theoretically possible.
Microtelecom Perseus – an HF SDR for the amateur radio market
Internals of a low-cost DVB-T USB dongle that uses Realtek RTL2832U (square IC on the right) as the controller and Rafael Micro R820T (square IC on the left) as the tuner
Modulation is defined as the process by which some characteristics like amplitude, frequency, and phase of a carrier signal are varied in accordance with a modulating wave.
Waterfall plot of a 146.52 MHz radio carrier, with amplitude modulation by a 1,000 Hz sinusoid. Two strong sidebands at + and - 1 kHz from the carrier frequency are shown.
A carrier, frequency modulated by a 1,000 Hz sinusoid. The modulation index has been adjusted to around 2.4, so the carrier frequency has small amplitude. Several strong sidebands are apparent; in principle an infinite number are produced in FM but the higher-order sidebands are of negligible magnitude.