Software-defined radio (SDR) is a radio communication system where components that have been traditionally implemented with hardware parts such as mixers, filters, amplifiers, modulators, demodulators, detectrs, etc. are instead replaced by software on a personal computer. Commercial manufacturers have begun to implement SDR techniques into commercial equipment which is far superior in performance to the previous hardware parts used.
Hams have discovered that cheap off the shelf unmodified consumer Digital TV broadcast dongles originally meant to be used to watch air on a computer can be used to create a cheap wide band rceiver. WHen the dongle is combined with free downloadable ham software on a PC, tablet or smart phone you have a wideband, all mode, communications receiver that will receive most of the amateur bands ( roughly 1 Mhz to 1.7 GHz). The performance is spectacular given the low price of around $20. The computer graphics in conjunction with the dongle and software also create a poor man’s spectrum analyzer for use in tuning transmitters, filters, and a host of other amateur related do-it-yourself building tasks.
The following link is to a SDR dongle page: https://www.rtl-sdr.com/