Software-defined radio

This article is written for beginners, higher level material can be found on using the links below

SDR# running in Windows 8

Software defined radio is the means of listening to radio waves using the combination of low level hardware, and software processing. While non-SDR techniques use physical hardware to demodulate FM and AM, In SDR raw data from the receiver gets sent into the USB port, and software is used to control the modulation. The only drawbacks of SDR are bandwidth constraints and processing. Since uncompressed unmodulated data is coming through a PC interface (USB mostly), a high speed connection and a computer with decent specs is required. A Single RTL-SDR May use up to 3.4MB/sec of USB bandwidth. CPU usage varies on the tuner software you use. Using software defined radio opposed to regular radio gives you the ability to see signals you would normally only hear. Your eyes can cover a very wide bandwidth of signals on a computer screen, while your ears can only comprehend a single spoken conversation at a time.

SDR models

SDRs come in many shapes in sizes, from the $8 RTL-SDR to $2000 USRP(with daughterboards) There are also many SDR's such as the HackRF, USRP, FunCube, etc. When you buy an SDR, you get what you pay for. More expensive SDR's are purpose built for transmitting, and can handle things such as GSM spoofing, where an unencrypted GSM tower can be simulated to capture SMS in a cleartext fashion.


Two common types of RTLSDRs

A great beginner SDR is the RTL-SDR It features a receive frequency range of 25-1700MHz, 2.048MHz bandwidth and is priced low enough that beginners can see whether radio is a hobby they want to get into(R820T model).

RTL-SDRs come with various tuner chips, the most widely used tuner is the R820T, with the E4000 coming in a close second. The E4000 is a discontinued model and is usually priced higher than R820T models. R820T tuners are generally better for reception of trunked radio systems (digital police radio).

The RTLSDR began its life as a DVB-T OTA TV tuner (a tv tuner for europe). China began bulk manufacturing these TV tuners for the european market. It was later found that a hacked driver could be installed that let the user use the dongle as an SDR.

Currently R820T RTLSDRs can be bought for as low as $8 on eBay with free shipping R820Ts on eBay

There is a minor update of RTLSDR named R820T2, which is still compatible with the old one and improve performances of reception.

Funcube Dongle

As part of AMSAT-UK’s FUNcube satellite project, the FUNcube Dongle is the “ground segment”, or a radio receiver designed to allow anyone to try their hand at reception of satellites like FUNcube anywhere on Earth as part of a global educational collaboration project collecting information from space. The original has a frequency range of 64MHz to 1.7GHz. The Pro + has a frequency range of 150kHz to 240 MHz and 420MHz to 1.9GHz.


The HackRF is a DARPA funded project, it aims to provide innovation and research in the SDR field. It's much more powerful than the RTL-SDR, much more sensitive, covers a greater frequency range. The HackRF starts at $300, has both RX and TX functionality from 30 MHz to 6 GHz, bandwidth of 20MHz. During DARPA funding, beta units, the HackRF Jawbreaker were given to hundreds of testers for free. The production version was funded through KickStarter. The campaign was successfully funded on September 4th 2013 and is currently available for purchase

TX functionality on the HackRF is limited in power because RF theory says you must have an antenna length that corresponds to the wavelength you are sending. For example sending 5Watts over the wrong size antenna will cause a buildup of standing waves, and may fry the transmitter. To prevent this, the antenna build into the breadboard of the HackRF is only designed for ultra low power, good enough for experimentation, but not for serious broadcasting.


SDR's have endless possibilities. Because all demodulation is done in software, no new hardware is needed to decode, for example, a new digital voice mode. All the programmer has to do is write code to convert the raw samples from USB to the correct format data.


Full article

Modulation guide

Some frequencies are used by only one type of modulation. For example 88-108 is broadcast FM, the modulation used is WFM. Using the correct modulation can help you decide what type of signals you are listening to. The table below lists the most common modulations.

Frequency Modulation
120MHzAM (Airband)

Points of interest

MF(Shortwave Radio)

An upconverter is required to listen to these frequencies

HF (3-30MHz)

An upconverter is required to listen to most of these frequencies




Upconverters allow you to listen to HF bands with a converter box and your existing RTL hardware. Normally this wouldn't be possible because the lower limit of an RTL-SDR is 24MHz. This allows you to receive AM, mediumwave, longwave, CB, and shortwave radio on the cheap.

How upconverters work

Upconverters essentially resample everything from 0-50MHz, and retransmit it, using low power, at 100-150MHz.

So for example, if you wanted to listen to a channel at 7MHz, and your offset is +100MHz, you would tune your SDR to 107Mz to listen to it. This math calculation is made easier by the offset feature found in many different types of SDR software. So once you set your offset in the program you are using, you don't need to think about adding 100MHz.

Radio signals on HF are very small, so although 0-50Mz doesn't sound like it would have lots of signals and stations inside it, they contain a majority of advanced ham radio operators, worldwide news networks including North Korea and Russia, as well as stations like the buzzer(UVB-76)


Software is available for Windows, GNU/Linux and OSX.

SDR Software (The stuff you listen with)

Software Supported Modes OS Cost/License Features/Limitations
SDR TouchAM, WFM, NFM, SSB, DSB, CWAndroidFreemium, Closed supports RTL_TCP and USB OTG, FFT times out after 30 seconds on free version
glSDRAM, WFM, NFM, SSB, DSB, CW, DIG(?), SPEC(?), SAM(?), DRMAndroidFree, GNU/GPLUI for ghpsdr servers
SDR-RadioWindowsFree, ClosedFeature packed, harder to get set up
SDR-Radio v2WindowsFree possibly paid out of beta, ClosedBeta, Feature packed, supports RTL_TCP
SDR#AM, WFM, NFM, LSB, DSB, USB, CWWindowsFree, MS-RSL/MITMost popular and easiest to use
SDR-JDABWin,LinFree, GNU/GPL2Digital Radio app (dabstick-radio in the AUR)
HD-SDRWindowsFree, Closed
PowerSDRWindowsFree, GNU/GPLDoes not support RTL-SDR
GQRXAM, WFM, NFM, SSB, CW, AFSK1200Win,Lin,MacFree, GNU/GPLRuns on GNU Radio
GNU RadioWin,Lin,MacFree, GNU/GPLTools to build your own receiver. Versatile, larger learning curve.
LinradWin,Lin,MacFree, OSS
Studio1Paid,Closed SourceFormerly WRPLUS

Plugins for SDR#

Purpose Software Cost/License
Scan and log various frequenciesFrequency Manager + Scanner pluginFree, OSS
Tune SDR# to the approprate voice channelTrunking plugin for SDR#Free, OSS

HF decoders (shortwave 0-30MHz)

Purpose Mode Software OS Cost/License Common Frequency
Digital decoding on HFCW, Domino, Hell, MFSK, PSK, OLIVIA, RTTY, Throb, Thor, WWVFldigiWin,Lin,MacFree, GNU/GPL0-30MHz AM
HF amateur radio codecFreeDV, Codec2FreeDVWin,Lin,MacFree, GNU/GPL0-30MHz AM
Digital broadcast shortwaveDRMDreamWin,Lin,MacFree, GNU/GPL0-30MHz AM
Sending images over HF, commonly QSLsRobot, AVT, Scottie, MartinMMSSTVWindowsFree, OSS0-30MHz AM

ADS-B Airplane tracking software

Purpose Mode Software OS Cost/License Frequency
Plane-spotting, decoding ADS-BADS-BADSB#WindowsFree, MIT1090MHz AM
Decodes ADSB# data + Plot planes on a mapAVR-BEASTVirtualRadar ServerWindowsFree, OSS1090MHz AM
Alternative to ADS-B#ADS-BRTL1090WindowsFree, Closed1090MHz AM
Decodes ADSB (S Mode)ADS-BDump1090LinuxFree, BSD1090MHz AM

APRS (like GPS, hams use it)

Purpose Mode Software OS Cost/License Frequency
Amateur packet radio incl. GPS locationAX.25, AFSK, APRSAFSK1200Win,Lin,MacFree, GNU/GPL144-145MHz FM, Varies

Trunked Radio (police fire and EMS in urban areas)

Purpose Mode Software OS Cost/License Frequency
Decode control channel dataP25, EDACS48, EDACS96, Motorola, MPT1327UnitrunkerWindowsFree, ClosedUsually 400MHz, 760,860,933MHz FM
Decode control channel dataLTR, MPT1327, Passport, P-25 Phase 1, Fleetsync II, LJ1200, MDC-1200, Tait 1200SDRTrunkWin,Lin,MacFree, GPL3Any
Decode digital voiceP25p1, ProVoice, X2-TDMA, DMR/MOTOTRBO, NXDNDSDWin,Lin,MacFree, BSDUsually 400MHz, 760,860,933MHz FM
Decode LTR dataLTRLTR AnalyzerWindowsFree, Unknown~400-500MHz

NOAA weather satellite

Purpose Mode Software OS Cost/License Frequency Other
Decode NOAA WX imagesAPT, WEFAXWxtoIMGWin,Lin,MacFreemium, Closed~137MHz FMTrack Live
Decode NOAA SatelliteAPTAtpdecLinuxFree, GPL~137MHz FM

Television (non digital kind)

Purpose Mode Software OS Cost/License Frequency
Analog television decoder, no soundNTSC, PALTVSharpWindowsFree, OSSAny AM (including ISM security cameras)


Purpose Mode Software OS Cost/License Frequency
Decode various pager formatsPOCSAG, FLEXPDWWindowsFree, GNU/GPL35-45MHz, 152-160MHz, 455-463MHz, 928-932MHz FM
Decode various including pagerPOCSAGmultimon-ngLinuxFree, GPLvarious

Audio routing software

Purpose Software Cost/License
Routes audio from the receiver to a decoder software aboveVirtual Audio CableShareware, Closed $35
Routes audio from the receiver to a decoder software aboveVB Audio CableDonationware, Closed ($5 donation gets you more cables)

Video Tutorials

SDRSharp (Windows)

GnuRadio (Linux)

Trunking police scanner (Windows)

Other SDR resources

/r/RTLSDR on Reddit

Scanner centric SDR discussion forums on RadioReference

FCC website, every RF transmitter registered registered with the FCC resides in this database

An online SDR receiver that covers the HF band (shortwave, worldwide transmissions)

2011 US frequency allocation chart PDF

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