Power Modules Provide Solutions For L/S-Band

Nov. 12, 2007
These compact power solutions modules (PSMs) help to simplify the addition of high-power amplifier stages to pulsed RF radar systems at L-band and S-band frequencies.

Pulsed radar systems consume power, in both operation and from the RF devices in the system. To feed the power needs of modern pulsed RF radar systems, a series of Lband and S-band power solutions modules (PSMs) has been developed by the Power Products Group of Microsemi (www.microsemi.com) for pulsed radar applications. These PSMs, currently available in bands of 1200 to 1400 MHz, 2700 to 3100 MHz, and 3100 to 3400 MHz, provide about two to three times the output power of single transistors currently on the market. They are designed for ease of installation using a "plug-and-play" approach that allows customers to directly drop these PSMs into their systems without further impedance design or matching. The high output power and efficiency of the modules is designed to allow these radar systems integrators to rethink their design cycle times and significantly reduce the size of the power amplifier by more than 50 percent, with dramatically improved production turnon yield.

Discrete high-power Class C silicon bipolar transistors are widely used for L-band and S-band pulsed radar applications. These discrete devices generally deliver about 200 to 370 W output power at L-band (1200 to 1400 MHz) and about 100 W output power at Sband (2700 to 3100 MHz). Because radar system-level requirements are generally in the multi-kilowatt range, those requirements must be met with multiple- device designs. In some cases, radar customers might design a 1- or 2-kW power-amplifier module as a basic building block and then combine several of these modules to achieve the final required output power for the system.

Unfortunately, this traditional approach involves large size and complexity as well as design time on the part of the customer to develop the modules. Combining multiple discrete devices often trades off efficiency for the sake of reaching a required final output level. In addition, developing matching networks is not trivial for making a transformation from the device-level impedance of about 2 Ω to the system characteristic impedance of 50 Ω. A great deal of RF transistor tuning is required in both engineering and production in order to match and combine the contributions of the individual discrete devices in reaching the final required output power.

As an example, one of the more common discrete-device configurations at L-band for a 2-kW building-block module is to use a total of 21 220-W discrete transistors, such as the model 1214-220M from Microsemi, in a 1 driving 4 driving 16 configuration (Fig. 1). The terminal impedance of this kind of discrete device is in the range of 1 to 2 Ω, requiring customers to design external input and output matching circuits to match these devices to the 50-Ωcharacteristic impedance of the system. In addition to requiring design skills, such tasks are time consuming. One the individual transistor is matched to 50 Ω, it is still necessary to design both a multiway power splitter at the input and a multiway power combiner at the output to drive the inputs and combine the outputs. After such a large number of splits and combinations, the overall module efficiency drops from about 50 percent down to about 35 to 40 percent because of the insertion loss of the 16-way power combiner. In addition, the size of such modules tends to be very large; the biasing network and low frequency filtering circuitry for sixteen transistors adds another dimension to the overall module complexity and labor intensiveness for production assembly and tuning.

The PSMs represent a practical solution to the design time and effort of combining discrete transistors to attain a certain output power at L-band and/or S-band frequencies. In addition, customers can also achieve cost savings in both the design and the manufacturing phases of their system amplifiers while providing considerably higher efficiency, reduced power amplifier size, and better system reliability in mission critical applications. Customers can use just one of 550-, 700-, or 800-W PSM to replace as many as four 220-W transistors commonly used in parallel at the output of L-Band power amplifiers. Fig. 2 shows a 2-kW amplifier designed with a model 1214-800P PSM.

The L-Band PSM Series consists of three models: the 1214- 800P, 1214-700P1, and 1214-550P. They feature 50-Ω matched input and output powers across the 1200-to-1400- MHz band with high output power for pulsed radar applications. The high-performance Class C modules deliver peak power outputs of greater than 550, 700, and 800 W at 50- percent collector efficiency at L-band frequencies, using 300- s pulse-width waveforms at 10-percent long-term duty cycle. The modules require no additional tuning or complicated impedance matching.

The PSMs incorporate proprietary Microsemi chip designs as well as effective power-combining techniques and advanced state-of-the-art automated assembly and testing. Their design and manufacturing advantages result in superior performance in power output, gain, efficiency, and footprint, while achieving outstanding module consistency and repeatability in high volumes.

The benefits of the PSMs are many, including near "plug-and-play" ease of use, with input and output impedances already matched to 50 Ω; a significantly reduced design cycle as a result of the impedance-matched modules; reduced system size and complexity, since fewer stages of the miniature PSMs are needed for a given output-power level; and improved system performance by merit of the high efficiency, reliability, and repeatability of the PSMs. In addition, the use of the prematched and tested compact PSMs eliminates system production and tuning time; the use of the PSMs can greatly increase production yield and reduce transistor scrap; and reduce system component inventory. In addition to standard PSMs, custom configurations are also available.

The NPN silicon bipolar junction transistor used in the PSM is designed and fabricated at Microsemi PPG - RF Products Division. The transistor has an interdigitated geometry with tight emitter-to-emitter pitch to increase the ratio of the emitter periphery to the base area, which is about 8 mils for this device. The emitter periphery and epitaxial material were chosen to provide nominal power of 100 W per chip biased at 50 V. Double-layer gold metallization is used to lower the output capacitance (COB) and also provide excellent mean time to failure (MTTF) at L-band frequencies. Nichrome emitter ballast resistors are used for enhanced linearity.

The transistor chips are attached to a 40-mil-thick metalized beryllium-oxide (BeO) substrate over a 60-mil-thick copper- tungsten (CuW) flange. The packaged transistors are internally matched with input and output metal-nitratemetal (MNM) capacitors also fabricated at Microsemi PPG-R. The input matching network consists of a twostage lowpass impedance-matching transformer designed by using the series inductance of bond-wires and the capacitance of shunt MNM capacitors soldered to the metalized ground plane. The output matching network consists of shunt inductive bond wires connected from the isolatedcollector/ die-attachment area to DC blocking capacitors which are also mounted on the metalized ground plane. All bond wires are straight and in-line, which allows fully automatic wire bonding for mass production and consistency. A total of 369 wire bonds are used in one of the modules. Fig. 3 shows the inside of the singleended transistor. These transistors are hermetically solder-sealed for the highest reliability.

The single-ended input and output impedances achieved with the internal matching design are shown in Table 1. The source impedance, ZS, and load impedance, ZL, are measured with a microwave vector network analyzer (VNA) using a through-reflect-line (TRL) calibration and are oriented away from the transistor (Table 2).

The L-band PSMs are built on RT/Duroid copper-backed printed-circuit- board (PCB) material from Rogers Corp. (Rogers, CT). The small size of the 1214-800P PSM, at 3.2 X 2.0 X 0.21 in. (81.3 X 50.8 X 5.3 mm), makes it ideal for customers with constraints on system mechanical dimensions. The surface of the PCB is electroplated to prevent oxidation of the copper conductive traces.

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The L-band PSMs incorporate Wilkinson circuitry for power combining and dividing. The low input and output impedances of the single-ended transistors are first transformed to an intermediate impedance of 25 Ω. This is then transformed to 50 ohms through the Wilkinson divider/combiner.

The power combining technique used in this power amplifier is a Wilkinson divider/combiner. Impedances of the input and output of the single-ended transistors are first transformed to a 25 ohms intermediate impedance. The two RF choke sections are chosen for a quarter- wavelength length at 1300 MHz. Two 50-Ω high-power AlN resistors, one for each side of the divider/combiner circuit, are used to provide isolation between the two single-end transistors. To achieve higher output power and high efficiency, the transistors are configured in common base mode and biased for Class C operation. Fig. 4 shows an example of the 800-W PSM.

To evaluate the performance of the PSMs, a text fixture was built with SMA connectors attached to the input and output of the RF terminals. Two highvoltage 4000-F storage capacitors are also soldered to the biasing circuits, one on each side of the PSM. Finally, a heatdissipating aluminum fin is mounted on the bottom of the PSM and an aircooling fan is used to control PSM thermal dissipation during testing.

The 800-W PSM was tested with signals at 300-s pulse width and 10-percent duty cycle biased at 50 V (Fig. 5 and Fig. 6). The output power was measured at the middle of the pulse, or 150 s into the pulse. As Fig. 5 shows, input drive power of about 110 W resulted in 800 W output power, corresponding to 8.6 dB power gain at 1400 MHz. At 1200 MHz, 893 W output power was measured for the same input drive level, corresponding to 9.1-dB gain. The output gain flatness for this PSM is better than 0.5 dB when measured with this fixed input drive power. For the 110-W input power level, the collector efficiency is around 50 percent. Fig. 7 shows a snapshot of a typical pulse shape at 1300 MHz with typical amplitude droop of under 0.3 dBindicating excellent thermal design. The return loss from 1200 to 1400 MHz is better than 12 dB.

Because of increasing demand for Sband pulsed radar devices, the PSM design concept was also applied to three platform products covering the frequency range from 2700 to 3400 MHz, the 200-, 300-, and 180-W models 2731- 200P, 2729-300P, and 3134-180P, respectively (Table 3).

At Microsemi, thermal die simulation and thermal scan analysis guide die designers to eliminate hot spots, producing thermally balanced die with low junction temperatures. Fully automated die attach, wire bonding, and assembly assures precise construction and repeatability. As a result, these devices and modules meet the critical requirements of phased-array radar applications with high lot-to-lot performance repeatability across volume production.

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