Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.
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Tutorial Challenges And Solutions For Testing AESA Radar T/R Modules AESA radar systems are extraordinarily versatile, but their complexity — requiring precise alignment of hundreds or thousands of T/R modules — can present significant measurement difficulties. By Darren McCarthy T he active electronically scanned array (AESA) architecture has redefined the definition of what a radar system is and can do, turning it into a multifunction powerhouse that makes its predecessors pale in comparison. It has also changed the way radars must be characterized, from component-level tests through complete system verification. The most critical AESA radar system measurement is arguably evaluation and optimization of the phase and amplitude uniformity of its transmit/receive (T/R) modules. This is no simple task in a system containing hundreds or thousands of T/R modules and perhaps 100,000 components, all orchestrated by millions of lines of software code and complex communications schemes that require precise timing. Such a daunting task is well beyond what traditional, discrete test equipment can perform in a reasonable amount of time, which makes this approach unacceptable in a production environment. Rather, a highly integrated, automated system is required that can reduce alignment time for all of the T/R modules from weeks to minutes or seconds per module. To illustrate why this is so, consider that an AESA radar T/R module consists of digitally controlled microwave switches, phase shifters, variable gain amplifiers, and other components that can interact, so every control state of every module must be tested and verified to be within prescribed limits. Accuracy to within a few tenths of a decibel and a few degrees of phase is required when measuring multiple states in each module, and any errors produced by a misaligned element must be removed through calibration. Test systemcreated insertion loss, impedance mismatches, and other artifacts must be de-embedded from the results. This requires a test routine (test case) composed of more than 25,000 measurements that translates into about 2.5 million measurements for even a 100-element AESA radar. It can certainly be performed with an ATE (automatic test equipment) system created from discrete instruments and proprietary software. However, not only is creating such as system a tedious task, its overall performance and accuracy are highly subjective, making increased measurement uncertainty likely and repeatability difficult to verify. In addition, the speed and accuracy at which this or any ATE system can make measurements are directly dependent not just on measurement hardware but on the quality 32 Electronic Military & Defense ■ www.vertmarkets.com/electronics of the test routines as well. It is essential that measurements of critical parameters "sample" many points during the measurement to provide adequate test confidence that the modules will perform at the desired levels. Taken together, these factors and the increasing deployment of Figure 1: An AESA radar showing the AESA radars have created T/R modules in reference to the antenna the need for ATE systems aperture. (Source: radartutorial.eu) dedicated exclusively to testing T/R modules at speeds orders of magnitude faster than a "homebrew" system can provide. Examining The AESA A passive electronically scanned array uses a single signal source and splits it into hundreds of paths, selectively delaying some and sending all of them to individual antennas. By contrast, an active electronically scanned array places signal sources and receivers at each element. In either case, the signals from each antenna overlap in space, and the interference patterns between them are controlled so that signal strength increases in certain directions and decreases in others, creating a radiation pattern. An electronically scanned array can also produce multiple active beams so that general scanning can be conducted in space while simultaneously targeting a specific object with another beam. With the emergence of highly integrated GaAs (gallium arsenide) MMIC (monolithic microwave integrated circuit) power amplifiers, the size of transmit and receive elements was reduced dramatically, producing a single functional block — the T/R module. This allows every module to operate on a different frequency, as each module generates and radiates its own independent signal, providing the ability to produce multiple sub-beams that can track multiple targets. It is also possible to change operating frequency and modulation characteristics with each pulse and many other variables. The "secret sauce" in the AESA (other than the T/R modules themselves) is the ability to perform digital beamforming