While not really a topic important to new hams, Software Defined Radio (SDR) crops up often in amateur radio publications and advertising so we should at least introduce the idea here.
As its name suggests, SDR involves software working in radio equipment. More than just being used for operator interface and general control, SDR software specifically replaces the functions of signal processing hardware employed by traditional radio circuits.
SDR greatly simplifies radio circuitry by replacing the functions of hardware oscillators, mixers, filters, modulators/demodulators, and detectors with software. Since software is easily changed this also means that radio functionality may easily be improved or enhanced, or allow for new modes, protocols, and interfaces to other devices by reconfiguration or reprogramming.
SDR may be used on radio transmitters but the most common implementation is with receivers. Most of the popular and available SDR products are receivers and most employ direct digital conversion techniques.
One of the more defining characteristics of SDR is the user interface (integrated or PC display) waterfall display and menu-driven controls interface.
There are various SDR interfaces out there, many (most?) are free applications. These will typically have a waterfall display showing the entire receiver passband, band/frequency controls, filtering and other features (volume, AGC, noise, mode), and display controls/customization, along with recording and playback capability.
Current SDR use in amateur radio is typically for the HF bands; stand-alone modules or dongles running on PCs are commonly found and complete transceivers are quite popular although more expensive than traditional HF rigs (see references below). As of this writing, the author is unaware of any widely-available VHF/UHF FM mobile or handheld SDR transceivers although they may be commercialized in the future (Yaesu promotes a multi-band multi-mode hand-held receiver coming soon).
One useful aspect to a wide-band SDR receiver (such as RTL-SDR dongle) is that, in addition to amateur radio bands, you can also receive a wide variety of other radio signals (general radio, air traffic control, public safety radio, ADSB, ACARS, trunked radio, P25 digital voice, POCSAG, weather balloons, APRS, NOAA APT weather satellites, radio astronomy, meteor scatter monitoring, DAB). Cost is also attractive; a $20 dongle plus free software and your own antenna to receive most all RF signals. Add a $60 up-converter for better performance on HF bands.
While we’re at this, let’s mention an extension of SDR called cognitive radio (CR). CR exploits the capability provided by SDR to add an intelligent, adaptive nature to operations. It is capable of learning how to most effectively use its features and re-configuring modes of operation to be the most effective and efficient. This is a new and even more advanced topic so we won’t go beyond this brief note.
Modern technology with increased use of embedded software-controlled modules makes SDR not only possible but practical and inexpensive. Look for growing use of SDR by hams in the future.
SDR Reference links
Be aware that ARRL references on SDR are rather old and need to be updated; this technology is changing rapidly.
Wikipedia good basic info
KB6NU’s blog SDR for newcomers to amateur radio? Good place to start w/references
New to SDR? Excellent reference; detailed
List of SDRs manufacturers and models, relatively up-to-date with rapidly changing tech
YouTube generic SDR demo
HF base/portable SDR transceiver manufacturers: