Software Defined Radio

A Software Defined Radio (or Software Defined Receiver) is a type of radio whose filters and demodulation are implemented in computer software rather then ICs or discrete electrical components as in a traditional radio, this is accomplished by sampling via an ADC.

SDRs can be direct sampling or sample an IF from a more traditional superheterodyne front end. This makes it possible to tap an SDR into the IF of any rig, using a configuration known as a panadapter.

Since their creation in the 1980s^{[citation needed]}, SDRs have become popular for a wide variety of applications. What makes an SDR useful is the software running on the computer can be tailored for a specific application (e.g DVB-T decoding) rather then relying on purpose made ICs and discrete circuits. The software implementation also means a single SDR can perform multiple purposes.

RTL-SDR

Amateur interest in Software Defined Radios began when it was discovered that the RTL2832U IC used in some DVB-T receiver dongles was indeed an SDR implementation, and that by installing a custom driver, it was possible to explore the entire receive range of the chip and apply any form of demodulation in software.

RTL-SDR has since grown into a large community dedicated to using these cost effective SDRs for various applications, the RTL-SDR Blog now sells purpose made dongles using the RTL chip.

WebSDR

A WebSDR is an SDR which is hosted on the internet and can be used by multiple users at once, while traditional receivers can be found on the internet, SDRs are well suited for this use as multiple users can listen to their respective frequencies instead of taking turns controlling a single VFO.

The first WebSDR was installed at the University of Twente in 2008 and covers the entire 0-30 MHz band with no gaps, it's still the most popular WebSDR with over 1,000 listeners during peak times.

WebSDR (and the similar KiwiSDR) is now an entire network of internet connected receivers able to be used by anyone and run by volunteers.