New Plug-and-Play Power Solution Modules for L- and S-Band Pulsed Radar Applications
By Jerry Chang, Dir. of Radar Products, Microsemi Corporation

A new series of L-band (1200-1400MHz) and S-Band (2700-3100MHz, 3100-3400MHz) power solution modules (PSM) has been introduced by Microsemi PPG for pulsed radar application. These PSMs offer two to three times more output power over the single transistors currently available on the market and are designed with an extremely user-friendly plug-and-play concept such that customers can directly drop in and use them without further impedance design work. The higher output power and higher efficiency performance, along with the plug-and-play feature, can drastically simplify customers’ system complexity, rapidly reduce customers’ design cycle time, significantly shrink power amplifier size over 50%, and greatly improve production turn-on yield.

Customers’ Need: Multi- kilowatt Systems with 1KW or 2KW as Building Blocks
The discrete high power Class C operated Si Bipolar transistors have been widely utilized for L-band (1200 – 1400MHz) and S-Band (2700 – 3100MHz) pulsed radar applications. Output power levels of these discrete transistors are generally at about 200W to 370W for L-band and 100W for S-Band. However, customers’ radar system output power requirements typically are at the multi-kilowatts range, which is far above the power level of a single transistor. Customers would usually design a 1KW or 2KW module as the basic building block and then combine multiples of them to achieve the final required system output power.

Customers’ Challenges with Traditional Approach:
• Combine 220W transistor 1x4x16 – size, complexity, time, efficiency, performance
• Impedance transformation from 2 ohm to 50 ohm
• RF transistor tuning and adjustment in both engineering and production

As an example, at L-Band, one of the most commonly employed configurations for a 2KW building block module is to employ twenty-one 220W discrete transistors, such as model 1214-220M, and use a 1 driving 4 driving 16 configuration, as illustrated in Figure 1. The terminal impedance of this kind of discrete transistor is at the 1- 2 ohm range; customers need to design external input and output matching circuits to transform such low impedance to 50 ohm in order to be compatible with other RF components in the system. Such tasks require specific knowledge, skill sets, and experience in the RF/microwave field and are quite time consuming. Once the individual transistor is matched to a 50 ohm system, the customer needs to design both a multi-way power splitter at the input and a multi-way combiner at the output to accomplish driving the inputs and combining the outputs, respectively, of the stage of 4 and stage of 16 paralleled transistors. The overall module efficiency after such a high number of combinations decreases from 50% down to 35-40% because of the loss of the 16-way 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.

PSM Solution and Benefits
The new PSM Series is designed to provide substantial reductions in system design time, real estate, and complexity. Most importantly, customers can 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 the 550W, 700W, or 800W PSM to replace up to four 220W transistors that are commonly designed in parallel at the output of L-Band power amplifiers. Figure 2 demonstrates a 2KW amplifier designed with 1214-800P.

The L-Band Power Solution Module Series consists of three model types: the 1214-800P, 1214-700P1, and 1214-550P. They provide a “50 Ohms IN-50 Ohms OUT” fully matched across 1200-1400MHz band, high power amplifier stage for pulsed radar systems. These high performance Class C modules are designed for unparalleled performance, delivering peak power outputs greater than 550W, 700W, and 800W at 50% collector efficiency, under a pulse format of 300 microseconds, 10% long term duty cycle. Their user-friendly feature provides customers with plug-and-play capability that requires no additional tuning or complicated impedance matching.

The PSM product family uses Microsemi proprietary chip design, effective power combining, 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.

Power Solution Module Benefits
• Extremely easy to use – 50 Ohms IN-50 Ohms OUT – Plug-and-play
• Significantly reduced design cycle - no complex RF impedance matching work required
• Reduced system size– PSM devices are compact
• Reduced system complexity – fewer combining stages required; no matching necessary
• Improved system performance – high efficiency, reliability & repeatability
• Eliminates system production transistor assembly and RF tuning time
• Greatly improves production yield – reduces transistor scrap
• Reduces system components inventory
• Custom-designed PSM to customer’s specifications available

PSM Key Specifications & Design Approach
L-Band PSM key product features:

• Frequency Coverage: 1200 – 1400 MHz
• Medium Pulse Format: 300 us, 10%
• Excellent Output Power: > 550W, > 700W, > 800W
• High Power Gain: > 8 dB min
• Collector Efficiency: > 50%
• Operation: Class C
• Compact Size: 81.3 mm x 50.8 mm x 5.3 mm ( 3.2" x 2" x 0.21")

Device Fabrication
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 very tight emitter to emitter pitch to increase the emitter periphery to base area ratio. The emitter periphery to base area ratio of this chip is about 8mil. The emitter periphery and epitaxial material were chosen to provide nominal power of 100W per chip biased at 50V. Double layer gold metallization is used to lower the output capacitance (COB) and also results in achieving excellent MTTF for the L-Band frequency range. Nichrome emitter ballast resistors are used for better linearity.

The transistor chips are attached to a 40mil thick metalized Beryllium oxide (BeO) substrate over a 60mil thick CuW flange. The packaged transistors are internally matched with input and output Metal-Nitrate-Metal (MNM) capacitors that are also fabricated at Microsemi PPG-R. Input matching network consist of a two stage low pass impedance matching transformer design by using the series inductance of bond-wires and capacitance of shunt MNM capacitors soldered to metalized ground plane. Output matching consists of shunt inductive bond-wires connected from the isolated collector-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. All together there are 369 wire bonds. Figure 3 shows the inside of the single-ended 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 using TRL technique and are oriented away from the transistor.

Circuit Overview Example: 1214-800P
This Power Solution Module is designed on Roger Corporation’s RT/Duroid copper-backed boards. The compact size of the 1214-800P PSM – 81.3 mm x 50.8 mm x 5.3 mm (3.2" x 2" x 0.21"; length x width x height) – makes it very attractive for customers who have constraints on system mechanical dimensions. The surface of the board is also electro-plated to prevent the oxidation of the copper boards.

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. This impedance is subsequently transformed to 50 ohms through the Wilkinson divider/combiner. The two RF choke sections are set to be quarter-wave length at 1300 MHz. Two 50 ohms high-power AlN resistors, one for each side of the divider/combiner circuit, are used to provide isolation between the two single-end transistors. The isolation is more than 20 dB, according to computer simulation. To achieve higher output power and high efficiency, the transistors are configured in common base mode and Class-C biased. The 800W Power Solution Module described here is shown in Figure 4.

RF Measurements and Performance
To facilitate the demonstration of PSM performance, a test fixture was built where SMA connectors could be attached to the input and output of the RF terminals. Two high-voltage 4000uF storage capacitors are also soldered to the biasing circuits, one on each side of the PSM. And finally, a heat dissipating aluminum fin is mounted on the bottom of the PSM and an air-cooling fan is used to cool the PSM during test.

The performance of this 800W PSM tested under 300uS pulse width, 10% duty cycle biased at 50V is shown in Figure 5 and Figure 6. The output power is measured at the middle of the pulse, which is 150uS into the pulse for this case. As shown in Figure 5, 800W of output power is obtained with input power drive at about 110W, which corresponds to 8.6dB power gain at a frequency of 1400 MHz. At 1200 MHz, 893W of output power was measured at the same input drive, which is 9.1dB gain. The output gain flatness for this power module is less than 0.5dB measured at this fixed input drive. The collector efficiency is around 50% at Pin=110W. A snapshot of a typical pulse shape at 1300 MHz. is shown in Figure 7. The typical amplitude droop is under 0.3dB – an indicator of excellent thermal design. The return loss across the frequency of 1200 to 1400 MHz is better than -12dB.

PSM Family for S-Band Pulsed Radar Application
The demand for similar product for S-Band pulsed radar applications has been increasing. The PSM family also consists of three platform products that cover the popular 2700-3400MHz frequency range: 2731-200P, 2729-300P, and 3134-180P.

Design Philosophy and Mass Production Capability
There are many approaches to RF transistor construction. Microsemi’s philosophy and heritage is to deliver microwave RF products that provide consistent RF performance from device to device – today and years from now. Every aspect of the operation is aimed at the highest level of RF consistency. Consistency is at the heart of chip development discipline. Thermal die simulation and thermal scan analysis guide die design to eliminate hot spots, produce thermally balanced die with targeted low junction temperatures that permit consistent die operation and the utmost reliability. Fully automated die attach, wire bonding and assembly assures precise construction and tight tolerances of all critical elements and dimensions. Automated RF test guarantees repeatable RF performance of all standard catalog items as well as all custom transistors built to customer specifications. As a result, the consistency and repeatability of critical parameters for Phase Array Radar (PAR) applications, such as output power, gain, and insertion phase from lot to lot are able to achieve extremely tight distribution in volume production.

Conclusion
Power Solution Modules have been designed and fabricated at Microsemi PPG with a plug-and-play concept that has built-in impedance transformation networks with 50 ohms terminal impedance at both input and output. Users can easily drop the PSM into their system power module without any further impedance design work and enjoy the benefits of faster design cycle time, simplified power module complexity, reduction of system size, improvement in system reliability, elimination of production tuning, improvement of transistor yield, and reduction of inventory of components count in addition to the obvious enhancement of higher power and higher efficiency.
For more information, please contact www.microsemi.com or e-mail psm@microsemi.com

Microsemi CORPORATION
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