High-power levels are a reality in many high-frequency transmitter systems, and components suppliers are now being asked to supply products that can handle the heat in smaller housings.
Handling large amounts of RF/microwave power is part science, part imagination. The science exists in the form of thermal flow equations and thermalmechanical design software programs to calculate temperature rises in a wide range of electronic materials based on input power levels, dissipative and radiative losses, and thermal conductivity, among other parameters. The imagination helps to visualize the thermal flow through a system and, hopefully, to identify potential hotspots.
This article will consider the high-power levels associated with transmitters, such as terrestrial and satellite communications systems and in radar and electronicwarfare (EW) systems and a sampling of products developed for these systems. Such power levels are often in the range of hundreds to thousands of watts, and require the use of the largest transistors and vacuum electronics, such as traveling-wave-tube amplifiers (TWTAs), to deliver required power levels to a transmit antenna.
High-power signal generation in military radar systems has traditionally relied on vacuum electronic devices, such as magnetrons and TWTAs, although the US Department of Defense (DoD) and other international defense agencies have funded the development of solidstate alternatives capable of producing over 100 W average (continuous) and peak (pulsed) output power. Earlier this year, Microsemi Corp. (introduced its model 0405SC- 1500M 1.5-kW UHF transistor-based on silicon-carbide (SiC) substrate material. Designed for pulsed radars, the common-gate Class AB device delivers 1500 W output power from 406 to 450 MHz when operating with 300-s pulses at 6-percent duty cycle.
At higher frequencies, the NPT1007 transistor from Nitronex, which is based on gallium nitride (GaN) substrate material, is usable to 1200 MHz. It achieves 200 W output power at 900 MHz with 18.3 dB gain. The device combines the output power of two separate transistors housed within a four-lead Gemini package with 18.3 dB gain and 63 percent efficiency.
Although transistors have gained in power, tube amplifiers, such as the model dB-4522 TWTA from dB Control, continue to supply high power levels while shrinking in size. The dB-4522 operates from 11 to 18 GHz. It delivers 450 W CW output power from11.0 to 17.5 GHz and 400 W CW output power from 17.5 to 18.0 GHz.
Designers of passive components must also follow the trend of higher power levels in smaller packages, in order to help miniaturize commercial systems such as communications cellular base stations and military systems such as communications and radar systems on unmanned aerial vehicles (UAVs). Because of the small size of high-frequency passive components, such as hybrid couplers, dissipating heat becomes a major issue even at tens of watts. EMC Technology, for example, recently introduced a line of chemical-vapordeposition (CVD) fabricated diamond chip resistors and terminations for applications through 26.5 GHz. Resistor models are capable of power levels to 150 W through 12.4 GHz while terminations, such as the model CTO603D, can operate to 18 GHz with 80 W power. This 50-O termination measures only 1.65 x 0.89 x 0.38 mm.
The newest generation of Xinger passive components from Anaren Microwave includes hybrid couplers measuring just 0.25 x 0.20 in., with models capable of handling more than 180 W CW power at 1 GHz (see p. 97). They were tested and modeled with thermal analysis tools (see figure) to study the thermal flow through the components. Such studies revealed that the use of plated viaholes made a significant difference in lowering temperatures at high power levels.
For thermal studies at high power levels, component designers typically use programs from SolidWorks, Thermal Desktop, RadCAD, and FloCAD from C & R Technologies, Icepak from ANSYS, and FloTHERM PCB from Mentor Graphics. For example, highpower component and subassembly developer Micronetics has applied SolidWorks to its analysis of high-power PIN switches (see Microwaves & RF, May, 2010, p. 63). The firm's thermal management strategy aims at maintaining safe diode junction temperatures at high power levels.
Along with thermal simulations at high power levels, measurements are also an essential part of high-power design, and a long-invaluable tool has been the pairing of a microwave power meter and power sensor. A number of suppliers offer quality products in this area, for testing CW and pulsed power levels through millimeter-wave frequencies, including Agilent Technologies, Anritsu, Giga-tronics, Krytar, Ladybug Technologies , Rohde & Schwarz, and Boonton Electronics.