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The Linux SDR software stack

Software-defined radio moves the majority of radio processing into software, facilitating relatively inexpensive wide-band hardware interfaces to the electromagnetic spectrum, especially those frequencies below 3GHz. Pairing advanced SDRs with software-defined, -tuned, and -selected antennae yields dynamically optimal cognitive radio. Perhaps most famously, DVB-T television tuners built around the RTL2832U chip, available in USB-A form factor for less than $30 (particularly the Rafael Micro R820T2), can reliably provide 2MHz of RX bandwidth anywhere from ~30MHz to ~2GHz (still lower frequencies are supported via direct sampling). For $500, powerful units capable of tremendous bandwidth and range (as well as transmission capabilities) are available, and from there it's not that great a leap to building your own stingray—if the cops have 'em, so should you, doyouknowhatiamsayin?

A few of Your Humble Wikist's toys

On the more ballin' end of things, a tricked-out Per Vices Cyan will run you $290k before shipping.

Almost every SDR fundamentally works by receiving electromagnetic energy via metal antennae, running that through an amplifier ("analog gain", done to place the signal in the next stage's sweet spot), and sampling the result using an ADC. The latter will have a resolution in bits, and a sampling rate measured in samples per second (or Hz). These samples pass through a digital down converter circuit, a low-pass filter, and finally decimation, emerging as a (complex) baseband frequency range, and streaming out as I/Q (in-phase and quadrature) pairs.

I did a Youtube video about SDR as #001 in my DANKTECH series.


SDRs are available both as standalone units and attachments for a PC, where PC functionality will generally be required for processing and display. I will generally be discussing only the latter.

Hosted SDRs

The following SDRs all require an attached computer; they are not standalone devices. I have personal experience with those in green.

Device Tuner BW (MHz) Samples (M) ADC Tune (MHz) Xmit? FPGA? Bus VendID ProdID MSRP
RTL-SDRv3 R820TR2 2.4 sustained
3.2 peak
8 25–1750 No No USB2A 0bda 2838 $20
R820TR2 2.4 sustained
3.2 peak
8 25–1750 No No USB2A 0bda 2838 $20
E4000 2.4 8 65–1100
No No USB2A 0bda 2832 $35
SDRPlay RSP2 10 12 DC–2000 No No USB2B $170
SDRPlay RSP1A MSI001 6 14 DC–2000 No No USB2B $100
SDRPlay RSPDuo 2x10 2–10.66 14 DC–2000 No No USB2B $280
HackRF One RFFC5072 20 2–20 8 1–6000 Half No USB2Aµ $300
LimeSDR LMS7002M 61.44 2.5–61.44 12 30–3800 2x Cyclone IV USB3 1d50 6108 $300
LimeSDR Mini LMS7002M 30.72 30.72 12 10–3500 1x Altera MAX 10 USB3A 0403 601f $159
LimeSDR Micro LMS7002M 10 10 12 10–3500 1x Altera MAX 10 GigE $300
AirSpy R2 R820TR2 10 2.5–10 12 24–1800 No No USB2Aµ $170
AirSpy Mini R820TR2 12 6 12 24–1750 No No USB2Aµ $100
AirSpy HF+ 0.768 16 DC–31
No No USB2Aµ $150
bladeRF 2.0µ xA4 AD9361 56 61.44 12 47–6000 2x Cyclone V USB3B 2cf0 5250 $480
bladeRF x40 LMS6002D 28 40 12 300–3800 1x Cyclone IV USB3B 2cf0 5246 $420
Ettus B210 USRP AD9361 56 61.44 12 70–6000 2x Spartan 6
USB3B $1100
Ettus B200 USRP AD9361 56 61.44 12 70–6000 1x Spartan 6
USB3B $675
NI Ettus X410 USRP 400 1--8000 4x Xilinx Zynq Ultrascale+
2x100GbE $25000
Per Vices Noctar 250 125 12 / 16 DC–4000 1x Cyclone IV
4xPCIe $2500
Mirics MSi3101 MSi001 5 12 64–240
No No $85
FunCube Dongle Pro+ 0.192 16 150–260
No No USB2A 04D8 FB31 $200
PlutoSDR AD9363 20 61.44 12 325–3800 1x Z-7010 USB2Aµ 0456 B673 $150
SignalHound BB60C 27 0.3125–40 14 DC–6000 No USB3Bµ 2817 0002 $2900
XTRX CS LMS7002M 120 0.2–80 12 30–3800 2x Artix7 35T 2xmPCIe $299
XTRX Pro LMS7002M 120 0.2–80 12 30–3800 2x Artix7 50T 2xmPCIe $599
OsmoSDR E4000 58.8 4.2 14 52–2200 No Lattice FXP2 USB2A Never sold
ColibriNANO custom? 3 122.88 14 DC–55 No Altera MAX 10 USB3A $270
CaribouLite RFFC5072 2.5 4 13 30–6000 2x Half ICE40LP1K RPi Hat $138



A good antenna can have significant impact on SDR capabilities. An antenna can be made from any conducting material. Most antennae are best at a few frequency bands (though relatively frequency-independent antennae exist, including the spiral antenna (Rumsey's principle) and log-periodic (fractal)); monopole antennae are best when they're 1/4 as long as the desired wavelength, while dipoles ought typically total either 1/2 or 5/4 of a wavelength. An antenna has some degree of directivity (also sometimes referred to as gain): highly directional antennae can work with weaker signals, but must be properly aligned with the source/target. An antenna ought further be properly aligned for the desired signal's polarization, which is commonly linear (horizontal, vertical, or slant), circular (left- or right-handed), or elliptical/mixed.

The "V-dipole" configuration rotates the two sections through an angle less than the typical 180°; 120° is typically optimal for this application. V-dipole is an excellent configuration for VHF. Otherwise, most signals are vertically polarized, save ham radio, which is typically horizontal.

Antenna connectors

  • SMA (SubMiniature version A): 7.9mm coax, MIL-STD-348, #6 SAE hex nut, 50Ω
    • Reversed gender to make incompatible RP-SMA thanks to government nonsense
  • MCX (Micro coaxial): 3.6mm coax
  • MMCX (Micro-miniature coaxial): 2.4mm coax
  • BNC (Bayonet Neill–Concelman): best at less than 3GHz
  • Hirose U.FL: 2mm coax, snap-on, these break easily and piss me off


Low-level software

The Linux SDR software ecosystem is robust, but complex. There are multiple middleware layers available, providing generic access to various hardware devices. Many user-exposed applications support both middlewares, sometimes in addition to their own native hardware support. As a result, there can be multiple ways to specify a given piece of hardware in a particular tool.


At the lowest level live the various driver libraries for SDR hardware. It is atypical for SDR hardware to require (or provide) a Linux kernel driver; most appear to be implemented wholly in userspace atop raw USB devices (exceptions include the Mirics MSi2500 and the Ettus USRP). Indeed, the primary interaction most users will have with their kernel might be removing (and possibly blacklisting) the dvb_usb_rtl28xxu DVB driver autoloaded for the RTL-SDRv3 USB dongle (the rtl2832_sdr and rtl2832 kernel objects go with this driver, and ought—despite their names—also be removed). This driver prevents the RTL from being used with rtl_sdr, the userspace RTLSDR libraries. To blacklist it, create an entry ending in .conf in /etc/modprobe:

dank@vespula:~$ cat /etc/modprobe.d/blacklist-rtl8xxxu.conf 
blacklist dvb_usb_rtl28xxu
blacklist rtl2832_sdr
blacklist rtl2832

note that users of initramfs might need to rebuild or modify their initramfs to include this file. Adding a blacklist entry does not remove a loaded module; for that, use rmmod dvb_usb_rtl28xxu.

Userspace drivers

As noted above, most of these drivers work on raw USB devices. You'll need set up appropriate udev rules to give users permission.

Family Software Source Tools
RTL2832U rtl-sdr from OsmoCom rtl_test, rtl_sdr, rtl_power, rtl_tcp, rtl_fm, rtl_eeprom, rtl_adsb
HackRF libHackRF from Great Scott hackrf_cpldjtag, hackrf_debug, hackrf_info, hackrf_spiflash, hackrf_sweep
BladeRF libbladerf from Nuand bladeRF-cli, bladeRF-fsk, bladeRF-install-firmware
Airspy AirSpy One from AirSpy airspy_gpio, airspy_gpiodir, airspy_info, airspy_lib_version, airspy_r820t, airspy_rx, airspy_si5351c, airspy_spiflash
Ettus UHD from Ettus uhd_cal_rx_iq_balance, uhd_find_devices, uhd_siggen, uhd_cal_tx_dc_offset, uhd_image_loader, uhd_siggen_gui, uhd_cal_tx_iq_balance, uhd_images_downloader, uhd_usrp_probe, uhd_config_info, uhd_rx_cfile

Middleware layers

Following the proliferation of device-specific libraries, efforts have been made to unite them under one portable userspace API. Both of these layers use the hardware-specific drivers described above, and as of 2019, both can use the other as a device type. For maximum capabilities, I typically:


The Open Source MObile COMmunications project is the primary caretaker of the RTL-SDR effort, which has largely replaced their own (discontinued) OsmoSDR. In addition, the Osmocom GNU Radio blocks (gr-osmocom, GrOsmoSDR) provide a hardware abstraction over many SDRs, though expressed in the idioms of GNU Radio. GrOsmoSDR was the first major open source radio abstraction, and the first place many enthusiast SDRs saw support outside of their own drivers and tools.

GrOsmoSDR uses for its device specification comma-delimited list of solitary arguments and/or argument=value pairs. A full reference must be distilled from source. Use of a Soapy driver can be indicated via adding soapy=0 to the driver spec. License: GPL3


The Pothos dataflow processing project required SDR sinks and sources, and presents a cleaner API for projects outside the GNU Radio umbrella via libsoapysdr (a GNU Radio block, gr-soapy, has been built directly atop libsoapysdr). Soapy's native hardware coverage lacks a few minor devices supported by GrOsmoSDR, but it can wrap osmocom devices with the SoapyOsmo module (as noted above, gr-osmocom can likewise wrap Soapy devices via its Soapy driver). Soapy hardware modules are dynamic libraries linked in via dlopen() on demand.

Soapy uses for its device specification a comma-delimited list of argument-value pairs, but always takes as the first pair "driver=FOO". License: Boost


This appears to be a Windows thing, so who gives a shit?

High-level software


  • rx_tools is a reimplementation of several of the rtl_sdr tools (rx_fm, rx_power, and rx_sdr) using Soapy for portability.
  • GNU Radio, the 300-kilo orca of open source software radio. GNU Radio is not the most intuitive program ever written, but it has a nice GUI on the front, a rich wiki, active (and expert) development, and 4,000 component blocks. If GNU Radio can't do it, it probably can't be done. Whether you can figure out how to do it with GNU Radio is an entirely different question. GNU Radio has both Osmocom and Soapy native blocks, because of course it does.
  • Pothos seems to be trying to be GNU Radio, but processing? It's a truly expansive framework; I primarily know it as the source of (and impetus for) SoapySDR.


These programs allow one to explore a particular captured signal, either from a file or in realtime from an SDR. In addition, most allow for exploration of the RF spectrum via a waterfall plot and/or an FFT plot, as well as configuration of locally-attached (or even networked) devices. There are about 4,000 of these.

The following are more special-purpose:

  • inspectrum: offline (no capture, only recorded files), but detailed
  • rtl_433: SoapySDR consumer that seeks to decode 315/433/868/915MHz transmissions
  • URH: Universal Radio Hacker, good for investigating unknown protocols
  • IMSI-catcher: captures and decodes basic GSM traffic

Wideband scanners


  • quisk: color scheme made me want to vomit, exited immediately

I've done all my transmitting thus far with GNU Radio, and honestly haven't generally known what I was doing.

RF spectrum

The radio spectrum is managed by the International Telecommunications Union. It is typically understood to cover those frequencies up to 300GHz, though the ITU lays claim to all EM "propagated in space without artificial guide" under 3THz. Above 300GHz, the atmosphere becomes effectively opaque until the low infrared. Radio waves are generated by electrically-charged particles whenever they undergo acceleration, and are carried by photons, the gauge boson/quantum/force carrier of the electromagnetic force. All photons travel at the speed of light in their surrounding material, and have zero rest mass (but do acquire mass and momentum via movement). A wave's length decreases linearly as the frequency increases: a 300GHz wave has a corresponding wavelength of about 1mm, whereas a 30Hz wave has a wavelength of 10Mm (10 orders of magnitude in each case). Microwaves are just high-energy radio waves (roughly everything above 300MHz); they are generated in the same way.

The following historical terms are still regularly seen:

  • Medium wave: 526.5–1606.5kHz (Europe), 525–1705kHz (US), channels every 9kHz (Europe) or 10kHz (US)
    • Groundwaves propagate following Earth curvature, skywaves reflect off of the ionosphere
    • AM radio
  • Longwave: everything below 525kHz (approximately; no official definition). Sees groundwaves; skywaves are rare.
    • Carrier frequencies every multiple of 9kHz, 153–279kHz
  • Shortwave: 2–30MHz (approximately; no official definition). Sees skywaves.

The ITU defines 12 RF bands, starting at 1. Each band n begins at the wavelength 10n, and covers an order of magnitude of wavelength. The corresponding lower frequency is 3x108-nHz. Medium wave corresponds to the lower portion of band 6 (Medium Frequency). Longwave corresponds to bands 1 (Extremely Low Frequency) through 5 (Low Frequency), though most amateur radio takes place within band 5. Shortwave encloses the top half of band 6, along with band 7 (High Frequency).

License-free operation

Of particular interest are the ISM bands. These "Industrial, Scientific and Medical" bands are reserved for purposes other than telecommunications. Operation within these bands is generally allowed by any device which can tolerate arbitrary ISM interference (there is no regulatory protection).

License-free frequency bands include:

Frequency Range Bandwidth Center Allocation
6.765-6.975 MHz 30 KHz 6.78 MHz Local ISM
13.553-13.567 MHz 14 KHz 13.56 MHz Global ISM
26.957-27.283 MHz 326 KHz 27.18 MHz Global ISM
40.66-40.7 MHz 40 KHz 40.68 MHz Global ISM
433.05-434.79 MHz 1.74 MHz 433.92 MHz Europe, Africa, Russia ISM
863-870 MHz 7 MHz 866.5 MHz Europe SRD860
902-928 MHz 26 MHz 915 MHz Americas ISM
2400-2500 MHz 100 MHz 2.450 GHz Global ISM
5725-5875 MHz 150 MHz 5.8 GHz Global ISM
24-24.250 GHz 250 MHz 24.125 GHz Global ISM
61-61.5 GHz 500 MHz 61.25 GHz Local ISM
122-123 GHz 1 GHz 122.5 GHz Local ISM
244-246 GHz 2 GHz 245 GHz Local ISM

See Also