The use of switches and other components based on monolithic silicon-on-sapphire (SOS) integrated-circuit technology can provide the performance needed for modern military radios.
Military radio designers face the challenge of making smaller radios that can do more than their predecessors. Such radios must use high-performance, high-reliability components to improve performance, while also meeting high-power Joint Tactical Radio System (JTRS) requirements, but within tight budgetary constraints. Because of the cost pressures, military radio designers can be expected to count more on commercial components, not only to save bill-of-materials (BOM) cost but on the size of the final radio design. One possible solution for achieving higher radio performance levels with small size and low power consumption is the use of siliconon- sapphire (SOS) components, including RF switches, frequency synthesizers, prescalers and digital step attenuators.
By means of field-effect-transistor (FET) stacking techniques, SOS devices can handle the high-power requirements of modern JTRS radio designs. UltraCMOS SOS devices, including switches, from Peregrine Semiconductor, for example, are widely used in mobile wireless handsets, which have supported high-volume production and lower prices. But using such devices in military radio applications depends on how they compare to other technologies in terms of power consumption, size, and performance.
The two basic types of military tactical radios in development either follow legacy JTRS design approaches with new radio definitions, or employ COTS technologies in an attempt to achieve lower cost with considerably lower power consumptions than traditional JTRS radios. Newer designs also often adopt software-defined-radio (SDR) architectures to allow changes in modulation type and channel bandwidth under software control for in-field flexibility.
This new COTS approach moves away from the high power levels (greater than 10 W) that tend to characterize military radios and moves to a smaller form factor at power levels of about a 1 W. Legacy JTRS systems require high powerhandling capability because of the long distances between a mobile unit and its base station. The high transmit power levels imply larger amplifliers with higher power consumption and less talk time for a given portable power source. Dissipating the higher power levels also requires a larger form factor, generally following the format of the "brick" type military radios. Newer radios, such as DARPA's Wireless Network after Next (WNaN) radio, provide a great deal more flexibility at lower power levels. The WNaN platform is a unique, scalable, adaptive mesh network radio architecture that uses inexpensive yet flexible radios that rely on standard COTS components and target 1 W per channel.
Another emerging tactical radio example is based on a civilian cellular line-of-site (LOS) base station approach employing an existing cellular network infrastructure, or deploying its own mobile base station in an area that does not have a cellular network infrastructure. In general, these newer 1-W tactical radio designs allow manufacturers to minimize power and battery requirements in packages that are more compact than legacy radios but still able to handle the requirements for hostile operating environments.
By reducing the transmit wattage of the mobile unit, designers will be able to dramatically improve talk time and form factor. The latest trend in military radio design is to leverage commercial radio development and capability from the cellular market. With this approach, the RF portion of the design becomes more standard; what is lost, however, is the ability to encrypt unique frequencies and ensure clandestine operation. As a result, encryption will be forced onto the software. For many designers, the cost, size, and performance benefits of this approach far outweigh any concerns over relying solely on software to keep lines secure.
For the warfighter, communications requirements in the field are similar to what is available from the cellular market: voice and video communications, photography, mapping, global positioning, and shortrange communications between soldiers. There is also a growing need to access many different forms of data: information, pictures, video, and mapping in real time. Smartphone capabilities are increasing the communication and data rates, and the volume of information in the user's hand, providing further benefits for the soldier including intelligence, reliability, and speed.
While these new military radio designs are enticing, they are mostly in the development stage, with a few systems being tested as designers find the optimal mix of capabilities, weight, and talk time for each type of military radio application. In the meantime, soldiers in the fi eld are still relying on military radios based on depleted nature of the individual FETs. The net result is that each FET handles the same voltage by equal voltage division. By expanding this stacking technique, it has been demonstrated that 10 W continuous power under 8:1 VSWR can be supported without performance degradation.
For portable handheld radios, size and weight are critical considerations. Especially for next-generation military radios, integration will be a key factor in the radio design. Increased integration with external control circuitry brought on chip can allow for higher reliability as well. For example, if a design required a digitally switched attenuator (DSA), switch, and a mixer, they could all be integrated into a single UltraCMOS device. Figure 3 shows a comparison of technologies, with a PINdiode- based single-pole, double-throw (SPDT) switch on the left that requires 20 external components, and a single selfcontained monolithic SPDT UltraCMOS switch on the right.
As the demands on military radios become more complex, and the needs for high reliability and data-handling capability increase, designers are turning to commercial RF switching and highly integrated circuits (ICs) for higher performance and value. These types of devices are particularly suited for transmit/ receive switching in improvised-explosivedevice (IED) jammers and mobile radios. Radio-frequency IC (RFIC) solutions such as the PE 42440 and the PE42510A switches are being designed into highperformance military radios to address many of the newest design demands. They can also be used for RF path selection, switching in filter banks, and low noise amplifier (LNA) bypass. Silicon-onsapphire RFICs, such as the UltraCMOS SPDT, single-pole, three-throw (SP3T), and single-pole, six-throw (SP6T) highpower switches; digital attenuators; PLLs; divide-by-2, divide-by-4, and divide-by-8 prescalers, are currently being designed into several military radio applications and are working side by side with GaAs PAs in the radio front end.