Software Defined Radios


Software-defined radio or SDR is a radio communication system where components that have been traditionally implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded system. While the concept of SDR is not new, the rapidly evolving capabilities of digital electronics render practical many processes which were once only theoretically possible.


A basic SDR system may consist of a personal computer equipped with a sound card, or other analog-to-digital converter, preceded by some form of RF front end. Significant amounts of signal processing are handed over to the general-purpose processor in the computer, rather than being done in special-purpose hardware like electronic circuits. Such a design produces a radio which can receive and/or transmit widely different radio protocols based solely on the software used or updates to the software.


Software radios have significant utility for the military and cell phone services, both of which must serve a wide variety of changing radio protocols in real time.


In the long term, software-defined radios are expected by proponents like the SDRForum (now The Wireless Innovation Forum) to become the dominant technology in radio communications. SDRs, along with software defined antennas are the enablers of the cognitive radio.


A software-defined radio can be flexible enough to avoid the "limited spectrum" assumptions of designers of previous kinds of radios, in one or more ways including:


Spread spectrum and ultrawideband techniques allow several transmitters to transmit in the same place on the same frequency with very little interference, typically combined with one or more error detection and correction techniques to fix all the errors caused by that interference.

Software defined antennas adaptively "lock onto" a directional signal, so that receivers can better reject interference from other directions, allowing it to detect fainter transmissions.

Cognitive radio techniques where each radio measures the spectrum in use and communicates that information to other cooperating radios, so that transmitters can avoid mutual interference by selecting unused frequencies.

Dynamic transmitter power adjustment, based on information communicated from the receivers, lowering transmit power to the minimum necessary, reducing the near-far problem and reducing interference to others, and extending battery life in portable equipment.

Wireless mesh networks where every added radio increases total capacity and reduces the power required at any one node. Each node only transmits loudly enough for the message to hop to the nearest node in that direction, reducing near-far problem and reducing interference to others.



The term "digital receiver" was coined in 1970 by a researcher at a United States Department of Defense laboratory. A laboratory called the Gold Room at TRW in California created a software baseband analysis tool called Midas, which had its operation defined in software.

The term "software radio" was coined in 1984 by a team at the Garland, Texas, Division of E-Systems Inc. (now Raytheon) to refer to a digital baseband receiver and published in their E-Team company newsletter.

A 'Software Radio Proof-of-Concept' laboratory was developed by the E-Systems team that popularized Software Radio within various government agencies. This 1984 Software Radio was a digital baseband receiver.

In 1991, Joe Mitola independently reinvented the term software radio for a plan to build a GSM base station that would combine Ferdensi's digital receiver with E-Systems Melpar's digitally controlled communications jammers for a true software-based transceiver.


Perhaps the first software-based radio transceiver was designed and implemented by Peter Hoeher and Helmuth Lang at the German Aerospace Research Establishment in Germany, in 1988. Both transmitter and receiver of an adaptive digital satellite modem were implemented according to the principles of a software radio, and a flexible hardware periphery was proposed.


The term "software defined radio" was coined in 1995 by Stephen Blust.

One of the first public software radio initiatives was the U.S. DARPA-Air Force military project named SpeakEasy. The primary goal of the SpeakEasy project was to use programmable processing to emulate more than 10 existing military radios, operating in frequency bands between 2 and 2000 MHz. Another SPEAKeasy design goal was to be able to easily incorporate new coding and modulation standards in the future, so that military communications can keep pace with advances in coding and modulation techniques.


Amateur and Home Use


A typical amateur software radio uses a direct conversion receiver. Unlike direct conversion receivers of the more distant past, the mixer technologies used are based on the quadrature sampling detector and the quadrature sampling exciter.


The receiver performance of this line of SDRs is directly related to the dynamic range of the analog-to-digital converters (ADCs) utilized in the product. Radio frequency signals are down converted to the audio frequency band, which is sampled by a high performance audio frequency ADC.


First generation SDRs used a PC sound card to provide ADC functionality. The newer software defined radios use embedded high performance ADCs that provide higher dynamic range and are more resistant to noise and RF interference.


A fast PC performs the digital signal processing (DSP) operations using software specific for the radio hardware. Several software radio efforts use the open source SDR library DttSP.[15]


The SDR software performs all of the demodulation, filtering (both radio frequency and audio frequency), and signal enhancement (equalization and binaural presentation). Uses include every common amateur modulation: morse code, single sideband modulation, FM, AM, and a variety of digital modes such as radioteletype, slow-scan television, and packet radio. Amateurs also experiment with new modulation methods: for instance, the DREAM open-source project decodes the COFDM technique used by Digital Radio Mondiale.


There is a broad range of hardware solutions for radio amateurs and home use. There are professional-grade transceiver solutions, like the Zeus or the Flex Radio brands, home-brew solutions, like PicAStar transceiver, the SoftRock SDR kit, and starter or professional receiver solutions, like the FiFi SDR for shortwave, or the Quadrus coherent multi-channel SDR receiver for short wave or VHF/UHF in direct digital mode of operation.


It was been discovered that some common low-cost European digital TV dongles called DVB-T USB featured a Realtek RTL2832U controller chip and tuner chip of either the Elonics E4000 or the Rafael Micro R820T. With modified USB drivers these TV dongles can be used instead as a wide-band SDR receiver. These basic SDR’s cost around $20 and look like a USB thumb drive.


More recently the GNU Radio using primarily the Universal Software Radio Peripheral (USRP) uses a USB interface, an FPGA, and a high-speed set of analog-to-digital and digital-to-analog converters, combined with free software. Its sampling and synthesis bandwidth is a thousand times that of PC sound cards, which enables wideband operation.


The HPSDR (High Performance Software Defined Radio) project uses a 16-bit 135 MSPS analog-to-digital converter that provides performance over the range 0 to 55 MHz comparable to that of a conventional analogue HF radio. The receiver will also operate in the VHF and UHF range. Interface to a PC is provided by a USB interface, although Ethernet can be used as well. The project is modular and comprises a backplane onto which other boards plug in. This allows experimentation with new techniques and devices without the need to replace the entire set of boards. An built in exciter provides 1/2 W of RF transmit ability over the same range.[27]


WebSDR is a project initiated by Pieter-Tjerk de Boer providing access via browser to multiple SDR receivers worldwide covering the complete shortwave spectrum.


Equipment to Buy


There are far too many SDR’s available today on the Amateur Radio market to detail all makes and models.

Prior to a few years ago a huge majority of these SDR products were a box without screens or knobs and all control and use was tied into software running on your PC or laptop.

In Amateur Radio these earlier SDR’s were broken down into two main categories Receivers and Transceivers.


Receivers were much less expensive and typically ranged from as low as $20 dollars to over $500 for well built and feature rich models. The common interface into your computer is USB and depending on the model of SDR you selected they could be used on VHF to Microwave bands, or other models designed for HF use.

Lots of free software and licensed software that you could purchase was available for these receivers and often the software allowed for add-ons or plug-ins to enhance the software to perform more functions that you wanted to use. This included acting more like a scanner, or capturing SSTV images, or tracking airplane GPS transmissions, etc.

A few years ago I used the free software called SDR# (sharp). In the last six months I’ve migrated over to SDR-Radio version 2 which I find or be easier to configure and use with my various SDR USB receivers.

If you’re looking for SDR receiver recommendations the products from AirSpy and SDRPlay are designed by Amateurs for Amateur Radio and offer great performance for about the $150 - $300 range. Always be sure the SDR receiver you buy operates in the bands that interest you the most as their ranges often vary by models and brands.

More and more in just the last few years Amateur Radio products utilizing SDR features can’t be differentiated from traditional older radios with their knobs and screen. This is because manufacturers of traditional non-sdr radios are embracing the performance and flexibility in the SDR technology especially in the receiver functions of a transceiver radio. It becoming very commonplace to be reading the specs on the new HF rig and see that SDR technology is a big part of the radios internal functionality. The nice thing about this evolution is the the manufacture instead of requiring that you run the radio from a PC or laptop, are providing a hardware front end for the radio with traditional switches and knobs. One example of this is the Elecraft KX3 QRP transceiver. Even though this radio looks and feels traditional in its design using knobs and dials, the radio is described by the manufacturer as having a SDR architecture. Another example is the ICOM 7300 which features Direct-sampling SDR receiver architecture.

ICOM has a brand new HF transceiver called the IC-7610 that they are actively promoting as The SDR You Have Asked For featuring a lot of nice features like

  • RF Direct Sampling System
  • Independent Dual Receiver
  • Dual Digi-Sel
  • Large Colour Touch Screen
  • And High- Speed, High-Resolution Dual Spectrum Scopes

One of the greatest features of these SDR architecture radios is a wider selection of filters can be built into the hardware with better performance and for less money. Another great feature is firmware updates for the SDR radio adding more bands, more filters, and more functions to a radio even years after it was originally manufactured and sold. I bought my Flex 5000a SDR transceiver about 5-6 years ago. In the years since the 60m band was added to our Canadian band plan and a simple software update allowed me to receive and transmit in that band as soon as it was available.

So even if you are a traditionalist who wants a radio with switches, dials, and knobs, your next new radio might be a software defined radio even if you didn’t know it, from it’s appearance.


As a final note on SDR’s I’d like to mention KIWI SDR.

Online there are hundreds of publicly accessible SDR receivers located all over the world. To view, control, and listen to these receivers use can use websites like the Hungarian hosted -- --

KiwiSDR is one of the hardware platforms that Amateurs can purchase and then host themselves online via websites like

NORAC is looking into hosting our own Kiwi SDR receiver at a good location in the North Okanagan this would be a location quit for HF interference and available to our members and others for remote monitoring of the HF bands, especially is your QTH is troubled by high QRM.

This investment would likely be around $500 for the hardware and the current Kiwi SDR technology allows for up to 4 remote users at the same time each monitoring and controlling what band or portion of a band they want to monitor.

I encourage everyone to play around with the website, all you need is your computer, laptop, tablet, or phone and a connection to the Internet.