Military applications often push technologies to their limits. Applications such as electronic warfare (EW), radar, communications, and surveillance have made demands on RF/microwave technologies over the years, causing oscillator manufacturers to design sources with lower phase noise, and synthesizer developers to increase frequency switching speeds. Although it would be difficult to summarize the many trends in electronic technologies for military systems, a few trends are in evidence.
Levels of integration have increased with growing demands for greater packaging densities. Not only are oscillator and synthesizer suppliers being asked to produce sources with lower phase noise, but they must do so in smaller packages. Synthesizer suppliers such as Synergy Microwave and EM Research are responding with compact solutions. Synergy's DCO and DXO VCOs are supplied in packages measuring only 3 x 3 in. while the firm also uses them in synthesizers measuring just 0.6 x 0.6 in. EM Research recently announced its PN-1090 frequency synthesizer for Mode Select (Mode S), Traffic Alert and Collision Avoidance (TCAS), and Air Traffic Control Radar Beacon Systems (ATCRBS). The synthesizer delivers +7 dBm from 1030 to 1090 MHz and measures just 0.9 x 0.9 x 0.15 in.
Along with signal sources, analog signal-processing components, such as filters, must fit in smaller spaces. Surface-mount RF/microwave bandpass filters as small as 0.5 x 0.5 in. are available from Bree Engineering while lowpass, highpass, and bandpass filters of 0.74 x 0.44 in. or smaller are available from Mini- Circuits. Even active components for military systems are being designed into smaller packages. The recently introduced model K M W 1 0 3 5 amplifier from AR Modular RF (see figure) is a lightweight portable bandswitching module for single or multi-band VHF/UHF tactical radio equipment. It measures just 2.2 x 5.2 x 7.75 in. but delivers 50 W from 30 to 512 MHz.
The higher packaging densities of these components also requires good thermal management, especially in systems using multilayer circuits. This has prompted suppliers of printed-circuit- board (PCB) materials to develop products with improved coefficient of thermal expansion (CTE) in the z-axis direction to ensure the reliability of the plated through holes. Many PCB materials suppliers have responded with enhanced materials, such as Rogers Corp. with its RT/duroid 5870 and 5880 highfrequency laminate materials, Arlon Materials for Electronics Div. with its TC600 material, and DuPont with its Pyralux AP rigid flexible circuit boards. The 5870 and 5880 materials offer low loss with dielectric-constant performance levels of 2.33 and 2.20, respectively, and are ideal for stripline and microstrip. The TC600 material is engineered for excellent thermal stability for use in high-power amplifiers and antennas. The DuPont Pyralux material is well suited for rigid flexible circuits with good plated through-hole reliability.
Moving higher in systems integration, receiver modules such as instantaneous- frequency-measurement (IFM) receivers and digital-frequency-discriminator (DFD) units are also packing more functionality into compact housings while gaining in performance as a result of improved digital components, such as analog-to-digital converters (ADCs) and field-programmable gate arrays (FPGAs). The faster clock speeds and higher bit resolutions of these devices at higher frequencies and wider bandwidths enable key system subassemblies such as IFMs and DFDs to make accurate frequency measurements on pulses as short as a few nanoseconds.
Additionally, more military receiver designers are exploring softwaredefined- radio (SDR) architectures for their flexibility in handling a wide range of waveforms in tactical applications. In essence, an SDR digitizes signals shortly after the receive antenna and generates waveforms for transmission by means of a high-speed DAC, allowing it to be reconfigured for different modulation bandwidths and modulation types based solely on software. In addition to military radios, the SDR architecture is also being adopted in many military radio test platforms for its software-reconfigurable flexibility. Major test equipment suppliers, such as Aeroflex, Agilent Technologies, Keithley Instruments, National Instruments, and Tektronix have adopted either SDR approaches and/or modular measurement formats such as PXI or VXI in response to the often multifunctional needs of military receiver, transmitter, and component testing. As an excellent review of SDR technology applied to radio testing, Tektronix offers a 16-page application note, "Testing Modern Radios," in PDF form for free download from its web site.
As an example of a set of challenges faced by high-frequency test equipment suppliers, National Instruments offers a case study on a test system developed for laser tag production testing. The system was needed to measure and adjust optical sensitivity, test wireless RF link performance, and verify the operation of the user display. In addition, the military customer required a similar system capable of providing shock profiles to 200 g's force and being able to align optical units under test to within 0.01 deg. The solution was a PXI-based system including a combination high-speed digitizer and digital multimeter augmented by GPIB-controlled instruments for optical signal generation, switching, and power measurement. The optical test source (reference laser) was modulated by means of a custom FPGA.