SDR – Software defined radio
SDR, or “software defined radio” is a radio receiver/transceiver made in a way that part of the hardware work is done by software. The software program can be run inside a Personal Computer, embedded system, micro controller, etc.
In a perfect word, a SDR receiver would look like an antenna directly connected to a AD (analog to digital) converter.
However, we do not live (yet) in a perfect word – and the way “things” are vectoring these days we are not going to anytime soon – so the most common SDR receivers are built with a antenna connected to a band pass filter, a low noise amplifier, the mixer and local oscillator. The resulting intermediate frequency is then converted to digital by the AD converter and after that the software program is taking over doing his magic.
Sometimes there is no intermediate frequency and the RF signal is directly converted to digital (especially for lower bands).