Understanding the Software Defined Radio Receiver SDR

The software designed radio receiver, SDR uses software to perform many of the basic functions of the receiver - using software it is easy to reconfigure and use the software on many platforms and for many different functions

Software Defined Radio Includes:
SDR basics     SDR hardware architecture     How to buy the best SDR    

Software defined radio technology has advanced significantly in recent years. Advances in hardware mean that costs have fallen and performance has rise.

This means that software defined radios are now seen in everything from high end radio communications equipment, to the simple USB plug in modules available at very low cost.

The software defined radio, SDR, technology is able to provide some significant advantages over traditional hardware based radio designs. Using the power of digital processing, software defined radios are being used in many different applications in many different areas.

Basic SDR concept

The basic concept of the SDR software radio is that the radio can be totally configured or defined by the software.

In an ideal world the incoming signal is immediately converted to a digital format, and the signal is then processed totally digitally.

Conversely for transmit, the signal is generated digitally, and converted to the final analogue signal at the antenna.

This approach has the advantages that the radio can be totally reconfigured for a new application, simply by changing the software. Updates can be made to keep up with new modulation formats, new applications, etc, simply by updating the software.

It also means that a common hardware platform can be used across a variety of different products and applications, thereby reducing costs, whilst maintaining or improving the performance.

 Example of a software defined radio
Example of a software defined radio used in research and development

Software defined radio applications

The SDR software radio concept is applicable to many areas of use:

  • Mobile communications:   Software defined radios are very useful in areas such as mobile communications. By upgrading the software it is possible to apply changes to any standards and even add new waveforms purely by upgrading the software and without the need for changes to the hardware. This can even be done remotely, thereby providing considerable savings in cost.
  • Research & development:   The software defined radio, SDR is very useful in many research projects. The radios can be configured to provide the exact receiver and transmitter requirements for any application without the need for a total hardware design from scratch.
  • Military:   The military have made much use of software defined radio technology enabling them to re-use hardware and update signal waveforms as needed.
  • Amateur radio:   Radio hams have very successfully employed software defined radio technology, using it to provide improved performance and flexibility.
  • Other:   There are very many other applications that can make use of SDR technology, enabling the radio to be exactly tailored to the requirements using software adjustments.

There are many opportunities for considering the use of the software defined radio, SDR, concept. As time progresses and the technology moves forward, it will be possible to use the concept in new areas.

Self contained software defined radio
This software defined radio is self contained but the software can be updated by download from the manufacturer to improve the functionality

Software defined radio definition

Although it may sound a trivial exercise, creating a definition for the software defined radio is not as simple as it seems. It is also necessary to produce a robust definition for many reasons including regulatory applications, standards issues, and for enabling the SDR technology to move forwards more quickly.

Many definitions have appeared that might cover a definition for a software defined radio, SDR. The SDR Forum themselves have defined the two main types of radio containing software in the following fashion:

  • Software Controlled Radio:   Radio in which some or all of the physical layer functions are Software Controlled. In other words this type of radio only uses software to provide control of the various functions that are fixed within the radio.
  • Software Defined Radio:   Radio in which some or all of the physical layer functions are Software Defined. In other words, the software is used to determine the specification of the radio and what it does. If the software within the radio is changed, its performance and function may change.

Another definition that seems to encompass the essence of the Software Defined radio, SDR is that it has a generic hardware platform on which software runs to provide functions including modulation and demodulation, filtering (including bandwidth changes), and other functions such as frequency selection and if required frequency hopping. By reconfiguring of changing the software, then the performance of the radio is changed.

To achieve this the software defined radio technology uses software modules that run on a generic hardware platform consisting of digital signal processing (DSP) processors as well as general purpose processors to implement the radio functions to transmit and receive signals.

In an ideal world the signal at the final frequency and at the correct level would emanate, and similarly for reception, the signal from the antenna would be directly converted to digits and all the processing be undertaken under software control. In this way there are no limitations introduced by the hardware. To achieve this, the Digital to Analogue conversion for transmission would need to have a relatively high power, dependent upon the application and it would also need to have very low noise for receive. As a result full software definition is not normally possible.

SDRplay RSPdx software defined radio
SDRplay RSPdx software defined radio


The JTRS, Joint Tactical Radio System, is a software defined radio initiative that provided a major impetus to the development of software defined radio technology.

Aimed primarily at military applications, JTRS was aimed at improving improve interoperability between different wireless networks, field radios, and devices.

The JTRS initiative comprised of both software and hardware, SDR technology so that multi-mode, multi-band, and multi-functional wireless devices and network equipment could be developed. Using SDR technology the aim was that they could be dynamically reconfigured, enhanced, and upgraded through software updates and hardware reconfiguration.

JTRS was a particularly attractive proposition, especially for coalition style operations where forces from different countries may operate together. Radios could be re-configured to enable communications to occur between troops from different countries, etc.

Advantages and disadvantages of software defined radios

As with any technology thee are advantages and disadvantages to the use of software defined radio technology.

Advantages of SDR technology

  • It is possible to achieve very high levels of performance.
  • Performance can be changed by updating the software (it will not be possible to update hardware dependent attributes though).
  • It is possible to reconfigure radios by updating software
  • The same hardware platform can be used for several different radios.

Disadvantages of SDR technology

  • Analogue to digital converters limit top frequencies that can be used by the digital section.
  • For very simple radios the basic platform may be too expensive.
  • Development of a software defined radio requires both hardware and software skills.

Software defined radios are being used increasingly. As processing power becomes cheaper to implement, so SDR based radios are being increasingly used for high end applications and also increasingly they are moving into lower end radios as well.

One of the major advantages of SDR technology is that it can be configured to exactly meet the requirements of the user - small changes to the software can make the radio fit the requirements exactly. Also with open source software like the GNU software, it is becoming increasingly easy to implement.

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