A Software-Defined Radio (SDR) system is a radio communication system where components that have typically been implemented in hardware (i.e. mixers, filters, amplifiers, modulators/demodulators, detectors. etc.) are instead implemented using software on a personal computer or other embedded computing devices. While the concept of SDR is not new, the rapidly evolving capabilities of digital electronics are making practical many processes that were once only theoretically possible.
A basic SDR may consist of a computer (PC) 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, rather than done using special-purpose hardware. Such a design produces a radio that can receive and transmit a different form of radio protocol (sometimes referred to as a waveform) just by running different 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.
An ideal transmitter would be similar. A digital signal processor would generate a stream of numbers. These would be sent to a digital to analog converter connected to a radio antenna.
The ideal scheme is, due to the actual technology progress limits, not completely realizable, however.
Real analog-to-digital converters lack the discrimination to pick up sub-microvolt, nanowatt radio signals. Therefore a low-noise amplifier must precede the conversion step and this device introduces its own problems. For example if spurious signals are present (which is typical), these compete with the desired signals within the amplifier's dynamic range. They may introduce distortion in the desired signals, or may block them completely. The standard solution is to put band-pass filters between the antenna and the amplifier, but these reduce the radio's flexibility - which some see as the whole point of a software radio. Real software radios often have two or three analog "channels" that are switched in and out. These contain matched filters, amplifiers and sometimes a mixer.
SDR Hardware Local Oscillator Phase Noise And Spurs
Currently the Direct Digital Synthesizers (DDS) for deriving the internal local oscillator signals for tuning the SDR receiver hardwares, are notorious for generating spurious RF byproducts in the passband of the receiver. These spurs as they are called, can mask weak signals and make entire band segments in the RF spectrum useless.
The project was demonstrated at TF-XXI Advanced Warfighting Exercise, and met all these goals. There was some discontent with certain unspecified features. Its cryptographic processor could not change context fast enough to keep several radio conversations on the air at once. Its software architecture, though practical enough, bore no resemblance to any other.
The basic arrangement of the radio receiver used an antenna feeding an amplifier and down-converter (see Frequency mixer) feeding an automatic gain control, which fed an analog to digital converter that was on a computer VMEbus with a lot of digital signal processors (Texas Instruments C40s). The transmitter had digital to analog converters on the PCI bus feeding an up converter (mixer) that led to a power amplifier and antenna. The very wide frequency range was divided into a few sub-bands with different analog radio technologies feeding the same analog to digital converters. This has since become a standard design scheme for wide band software radios.
The project produced a demonstration radio only fifteen months into a three year research project. The demonstration was so successful that further development was halted, and the radio went into production with only a 4 MHz to 400 MHz range.
The software architecture identified standard interfaces for different modules of the radio: "radio frequency control" to manage the analog parts of the radio, "modem control" managed resources for modulation and demodulation schemes (FM, AM, SSB, QAM, etc), "waveform processing" modules actually performed the modem functions, "key processing" and "crytographic processing" managed the cryptographic functions, a "multimedia" module did voice processing, a "human interface" provided local or remote controls, there was a "routing" module for network services, and a "control" module to keep it all straight.
As a military project, the radio strongly distinguished "red" (unsecured secret data) and "black" (cryptographically-secured data).
The project was the first known to use FPGAs (field programmable gate arrays) for digital processing of radio data. The time to reprogram these is an issue limiting application of the radio.
This goal is achieved through the use of SDR systems based on an internationally endorsed open Software Communications Architecture (SCA). This standard uses CORBA on POSIX operating systems to coordinate various software modules. The SCA documentation is freely available at the JTRS website
The program is providing a flexible new approach to meet diverse warfighter communications needs through software programmable radio technology. All functionality and expandability is built upon the Software Communications Architecture (SCA).
The SCA, despite its military origin, is under evaluation by commercial radio vendors for applicability in their domains.
Uses include every common amateur modulation: morse code, single sideband modulation, frequency modulation, 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.
More recently, the GNU Radio using primarily the Universal Software Radio Peripheral (USRP) uses a USB 2.0 interface, an FPGA, and a high-speed set of analog-to-digital and digital-to-analog converters, combined with reconfigurable free software. Its sampling and synthesis bandwidth is a thousand times that of PC sound cards, which enables an entirely new set of applications.
In addition the HPSDR (High Performance Software Defined Radio) project uses a 16bit 135MSPS analog-to-digital converter that provides performance over the range 0 to 55MHz comparable to that of a conventional analogue HF radio. The receiver will also operate in the VHF and UHF range using either mixer image or alias responses. Interface to a PC is provided by a USB 2.0 interface.
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 exciter provides 1/2W of RF over the same range or into the VHF and UHF range using image or alias outputs. The HPSDR project is open-source for both hardware and software. A Wiki provides frequent updates as to project progress.
On the low-end (and low-cost): the SoftRock kit gives an easy entry into direct conversion shortwave receiver with software-defined demodulation.
Another low cost software defined radio for amateur use is the WonderRadio by SDR Technologies from India . This low cost (under $600) radio covers the entire HF amateur spectrum + 6m. SDR Technologies has also developed an improved $1500 version of their basic 1 Watt kit called the Wonderradio Pro that includes a built in computer,speakers and a 10 Watt linear amp (with an optional 100W linear Amp). The complete standalone transceiver requires an external monitor, keyboard and mouse. Both radios use the parallel port and the Flex Radio SDRPro software for control.
These are useful books: