Electronic Military & Defense Annual Resource

3rd Edition

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 that, in addition to having all of the standard benefits of digital control, can increase dynamic range, precisely control the aperture's multiple beams, and provide greater control of amplitude and phase. As the magnitude and phase of the RF signals fed to each element can be individually controlled, wavefronts can be achieved for nearly any desired shape. To illustrate this, Figure 2 shows an AESA radar with only two antennas for simplicity. In this example, two sine wave signals are fed to two omnidirectional antennas, each with the same phase and magnitude, so they are coherent. The power of the radiated beam is the sum of the two sine signals. The figure shows the relationship of the perpendicular direction in the two-element antenna, and behavior in this direction is shown in the left half of the figure. A specific side transmission, or side lobe (in this case 45˚), is shown in the right half. The blue and green parts of the drawings show the energy transmitted from the two antennas, and the red parts show the two superposed antenna signals. As indicated in the lower right, the side-lobe radiation causes a phase shift in both antenna signals that causes the red sum signal to be less intense compared to the sum signal without phase shift. In short, the antenna has a specific maximum (main) propagation direction (90˚) because the two signals fed to the antennas have the same phase and magnitude. The sidelobe beam is reduced because of the phase shift obtained with different propagation path lengths. The manner in which the array's sum signal decreases with the side angle depends on the distance between the antenna elements and operating frequency because of the relationship of phase shift to wavelength. If a small phase shift is applied between the signals, the aforementioned effects can be reversed: the 90˚ beam is reduced, and the 45˚ side beam is increased to its maximum. As the phase between the signals is increased, the beam moves slowly to 45˚, reforming the beam. It is possible to see now why phase and amplitude alignment of each T/R module is crucial and precise alignment critical. Very fast automated test systems are required to align an array of hundreds, sometimes thousands of elements to achieve target performance. An Example AESA Test System Standard ATE test systems, like the R&STS6710;, can be configured to characterize the 25,000 measurement results required for a typical T/R module in less than 4 minutes and make the typical 2,500 key measurements required in production in less than 15 seconds per module. The system covers frequencies from 1 Hz to 24 GHz and can measure S-parameters for all phase attenuation combinations, noise factor, RF output power, intermodulation distortion, and spurious emissions. Two modules can be tested at the same time with a single system, or up to eight T/R modules can be tested in parallel. It consists of only three major components to provide all measurements, including module 34 Electronic Military & Defense ■ www.vertmarkets.com/electronics Figure 2: The effects of digital beamforming. control and power supply: a vector network analyzer for RF measurements, a control platform for RF signal switching and conditioning, and a test platform for TRM control and supply. As most important aspects of defense radars are classified and proprietary, the system is designed so that the test system supplier does not require (nor have) detailed information about the modules to be tested (such as bandwidth and operating frequency). The C# source code for the test cases is included with the system so the user can compile complete test sequences of varying size, the shortest covering only the parameters required for "fine tuning" a component in development. The system can also be operated manually. The system can be expanded by either simultaneous testing of two T/R modules (which cuts test time per module by half) or by multiplexing four test fixtures, allowing eight modules to be tested. The latter approach allows devices to be inserted into other test fixtures while measurements are being performed to ensure continuous testing and for comparison testing of devices when multiple T/R modules are combined to form a larger functional block. Summary Phased-array radars, the most advanced example being the AESA architecture, are extraordinarily versatile systems whose sophistication is increasing every year. However, they are also immensely complex and thus present significant measurement challenges. Without the availability of ATE systems dedicated to the AESA's unique requirements, it would be impractical to make all of the measurements required of a modern AESA radar in a production environment. Darren McCarthy is the aerospace and defense technical marketing manager for Rohde & Schwarz America. He has worked extensively in various test and measurement positions for more than 25 years, including R&D; engineer, R&D; project manager, product planner, and business and market developer.

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