Software-defined radio
| Software-defined radio | |
|---|---|
| Abbreviation | SDR |
| What it is | A radio whose tuning, filtering and demodulation are done in software |
| Receive-only examples | RTL-SDR dongles, Airspy, SDRplay |
| Transceiver examples | FlexRadio, Icom IC-7300 and IC-705, Elecraft K4, ADALM-Pluto, HackRF |
| Typical software | SDR#, SDR++, GQRX, CubicSDR, WSJT-X |
| Price | From about $30 for a receiver dongle |
A software-defined radio, or SDR, does with a computer what traditional radios do with coils, crystals and mixers. An analog-to-digital converter samples a wide swath of spectrum, and software selects the signal, filters it and turns it into audio or data. The result is a radio whose features are limited by code rather than hardware, and whose most recognizable feature is the waterfall display, a scrolling picture of everything happening across a band at once.
Most new amateur transceivers sold today are SDRs inside, whether or not they look like one, and a $30 USB dongle can turn any laptop into a receiver that covers everything from the HF bands to the microwaves.
Receive-only SDRs
The cheapest way into radio is an RTL-SDR, a dongle built around a television tuner chip that hobbyists discovered could receive anything from about 24 MHz to 1.7 GHz. With a converter or a direct-sampling mod it hears HF too. Plug it into a computer, run free software such as SDR# or SDR++, attach an antenna, and you can:
- listen to local repeaters, aircraft, marine and weather radio before you're licensed;
- see where the activity is on a band and spot signals you'd never find by tuning;
- decode FT8, APRS, satellite telemetry and weather-satellite images;
- use it as a panadapter beside a conventional radio.
Better receivers such as the Airspy and SDRplay families add wider bandwidth, more dynamic range and continuous HF coverage.
SDR transceivers
On the transmit side, SDR has reshaped the market. Icom's IC-7300 brought a direct-sampling HF transceiver with a waterfall to the mainstream at a mid-range price, and most makers followed. FlexRadio and others go further, with radios that have no front panel at all and are run entirely from a computer screen, sometimes from the other side of the world over the internet. At the experimental end, the ADALM-Pluto and HackRF transmit from a few megahertz to several gigahertz at low power, for hams who want to write their own modems or work on satellites and microwave bands.
Why it matters
SDR made features that once cost thousands of dollars, such as brick-wall filters, noise reduction, spectrum displays and remote operation, into software updates. It made the weak-signal digital modes practical. And it lowered the price of listening to almost nothing, which has brought in a generation of hams who first heard the bands through a dongle.