SDR listening

What it is

A software-defined radio (SDR) receiver digitises a whole slice of spectrum and leaves the tuning, filtering and demodulating to software. Inexpensive USB SDR dongles, of which the RTL-SDR is the best-known example, put a wideband receiver in reach of anyone with a computer. You can listen with one, see everything in a band at a glance, and learn a lot about how signals look.

A window on the band

A traditional receiver hears one signal at a time. An SDR hears a whole window at once, and its width is set by how fast the converter samples. The software turns the window into a spectrum and a scrolling waterfall where each signal is a bright trace, then demodulates whichever one you pick. Moving the centre frequency slides the window, and you can record the whole window to listen to later. The signal chain behind this is in Direct conversion and SDR receivers.

Seeing everything is the big advantage: you notice a signal you were not tuned to, spot interference, and watch the band open and close. The cost is detail: a wide window shows less of each signal.

What you can listen to

Listening	What the SDR adds
Local repeaters, simplex and APRS on VHF and UHF	See at once which channels are active
Your own signal	Check it is clean and see how it sounds elsewhere
Beacons and weak-signal activity	A waterfall picks out signals you could miss by ear
Interference	Watch noise change as you unplug things; see RFI and EMI overview
Weather, aircraft and other services	Fun to explore; check the law on what you may monitor
Internet-linked receivers	Listen through another person's antenna and receiver

Many digital modes are decoded by software, so an SDR plus a computer can receive several signals in one window, each decoded separately. The same idea powers the panadapter built into many transceivers; see SDR transceivers.

Getting good results

• The antenna matters most. The small whip in the box is a compromise. An antenna cut for the band, outdoors and high, makes a bigger difference than any setting; see Receive loops and active antennas.
• Watch for overload. A strong nearby broadcast or transmitter can overwhelm a cheap converter, producing phantom signals across the window. Reduce the gain, add a filter for the offender, or move the antenna; see Receiver overload and intermodulation. More gain is not better.
• Expect some drift. Many low-cost dongles use simple timing crystals that drift as they warm up, so frequencies read slightly off until you apply a correction.
• Keep the computer quiet. USB cables and computer power supplies radiate noise. A short extension cable, ferrites on the leads and a little distance help.

Related

• Receivers, scanners and SDR donglesListening-only gear.
• Direct conversion and SDR receiversSkipping the IF with I/Q sampling.
• SDR transceiversSoftware-defined radios as full stations.
• Sensitivity, selectivity and dynamic rangeThe numbers that describe a receiver.
• Spectrum analyzersSeeing signal strength against frequency.
• ADCs and DACsConverting between analog voltages and numbers.