Part one outlines SDR requirements and architectures.
One of the best known of SDR suppliers to both military and civilian customers is the RF Communications Division of Harris Corporation. Harris, selected by the DoD's JTRS Joint Program Office for the Step 2B validation of the JTRS platform and the SCA specification for battery-powered, manportable SDR platforms, has leveraged its FALCON II AN/PRC-117F radio to create the JTRS prototype.
Having successfully demonstrated voice and data waveforms in 2001, Harris has been extensively involved in the development of SDRs and the evolution of the SCA standard. Among its SDR waveforms, Harris has developed 28 Mb/s offset quadrature phase-shift-keying (OQPSK), 60 Mb/s OQPSK, and 274 Mb/s OQPSK configurations. These custom waveforms, of course, exceed the performance of standard Falcon II radios which provide bits rates to 64 kb/s in AN/PRC-117F(C) configuration.
The company's single-channel RF-300M-HH JTRS SCA-enabled hand-held radio (Figure 3) is programmable with a variety of platforms including SINCGARS, HAVEQUICK II, VHF/UHF AM, and FM waveforms. A future optional software upgrade will provide APCO 25 Land Mobile Radio support for interoperability with civil authorities. The RF-300M-HH is fully compliant with JTRS and covers 30 to 512 MHz and provides an adjustable transmit power to 5 W. It can also be optionally equipped with a built-in Global Positioning System (GPS) receiver.
Spectrum Signal Processing offers a leading SDR development platform, the SDR-3000 Military Communications Rapid Prototyping Development Platform (MRDP). The platform consists of three main elements: an RF transceiver subsystem, a signal-processing section, and an SCA software environment.
The RF transceiver consists of CompactPCI cards from Digital Receiver Technology that receive signals from 0.5 MHz to 3 GHz and convert them to digital IF bandwidths as wide as 30 MHz, and also generate analog transmit signals from 40 MHz to 2.9 GHz based on digital IFs as wide as 16 MHz. The transceiver platform also uses GPS as an absolute time reference, and performs slow and fast frequency hopping at rates to 5000 hops/s when triggered with external sources.
3. The single-channel RF-300M-HH JTRS SCA-enabled hand-held radio operates from 30 to 512 MHz and supports a variety of existing radio platforms including SINCGARS and HAVEQUICK II radios. (Photo courtesy of Harris Corp.)
The modular architecture of the SDR-3000 offers insight into test needs for the analog front-end circuitry. Although future SDR designs may cover even greater frequency ranges, this platform's receiver spans almost 3 GHz with modulation bandwidths as wide as 30 MHz. Within that spectrum, the possible combinations are endless.
An ideal test solution would perform measurements on every possible SDR hardware/software combination or waveform. In reality, such a solution would require the test equipment manufacturer to program instrumentation for as many as 30 or more SDR waveforms (hardware/software combinations), in some cases based on modulation formats not yet created. Test equipment suppliers would need a crystal ball to properly prepare for SDR front-end testing. In addition, it is usually not sufficient to simply test a simple waveform, such as FM, as this almost never will exercise the radio hardware over its full capabilities. For example, the peak-to-average of an FM signal is very low, so it will not show clipping problems in power amplifiers.
A more practical solution to SDR front-end testing is to focus on the radio's analog/RF characteristics, which effectively end at the ADCs and digital IFs. As seen from the SDR-3000 example, even this amount of testing is not trivial, since the SDR platform's receiver and transmitter encompass such wide instantaneous bandwidths over extremely wide frequency spans.
Testing an SDR platform such as the SDR-3000 requires both a test signal source and a wideband signal analyzer. Both instruments must provide enough frequency range (0.5 MHz to 3 GHz) to cover the radio's range, with adequate instantaneous bandwidth to handle the different SDR modulation formats. The test signal source will be used to "exercise" an SDR's analog radio section. To do so, it must essentially emulate an SDR's transmitter, covering the full frequency range and controlling complex modulation formats, such as OQPSK.
The test signal source should be able to switch signal formats quickly, under software control, in the manner of an SDR, and provide stable, low-phase-noise signals, good amplitude control, and relatively fast switching speed. It should provide programmable signal bandwidths, narrow enough to emulate the 3 kHz or less of HF radio channel bandwidth, and wide enough to accommodate emerging high-data-rate, wide-channel radios operating in both terrestrial and satellite links.