Understanding SDRs And Their RF Test Requirements

Evaluating tactical software-defined radios (SDRs) poses no small challenges for test engineers. Even in just considering the analog hardware attributes of these radios, a test engineer is faced with enough bandwidth for multiple communications standards, the capability of transmitting and receiving signals at high data rates, and the capability for simultaneous operation of multiple radio channels. Even ignoring spread-spectrum operation, such as frequency-hopped signals, the analog portion of these radios resembles multiple radios and requires special test needs.

An SDR is not unlike a personal computer, wherein the function of the computer is defined by its software. In the same way, the functionality of the radio is defined by software loaded into the radio. The hardware must be relatively generic but extremely broadband, with the software controlling frequency, modulation, channel bandwidth, security functions, and waveform requirements. Although the concept of an SDR was developed by the military to meet the requirements for reliable and secure communications across different branches of the armed forces, there is increasing interest in the technology for commercial applications. FlexRadio Systems (www.flex-radio.com), for example, currently manufactures the SDR-1000, a commercial SDR for use by amateur radio operators. It incorporates a direct-digital synthesizer (DDS) for frequency agility and waveform flexibility from 12 kHz to 60 MHz.

For the purpose of this application note, however, the focus will be on tactical SDRs for military use. The United States Department of Defense (DoD) is driving the development of SDR technology through its $1 billion Joint Tactical Radio System (JTRS) program. The goal of the program is to replace traditional hardware radios with units covering 2 MHz to 2 GHz and beyond that can emulate any radio by changing software. Software upgrades sent via wireless networks will keep fielded JTRS devices current and compatible. In addition to the US DoD, most European defense agencies have SDR development programs similar to the JTRS program. Most development programs specify SDRs in a wide range of footprints, from compact, manportable units to vehicle-mounted and shipboard platforms.