RF Power Devices Meet Wireless Challenges Head On
The number and types of commercial wireless-communications technologies have exploded in the last 15 years, from comparatively simple systems using FDMA to the higher-order modulation schemes employed in GSM and CDMA and new standards and services such as WiMAX and ZigBee^&tm;. For semiconductor manufacturers serving these applications, they represent immense opportunities. For their designers, these opportunities are accompanied by the challenges inherent in developing products that achieve greater and greater performance while also being manufacturable in high volumes at low cost. Freescale addresses diverse wireless applications with families RF power devices that meet the unique requirements of each one.

WiMAX: A Ripple Or The Next Big Wave?
While advancements in cellular and WiFi technologies are essentially evolutionary and their missions clearcut, WiMAX has the potential to be truly revolutionary, streaking across the boundaries of specific applications to complement (or compete with) the functions performed by both. Wildly optimistic market research notwithstanding, it's too soon to tell how much impact WiMAX will have in unseating entrenched services.

Its capabilities, if fully realized, could allow it to replace wired T1 lines and microwave links in cellular backhaul, provide a wireless alternative to cable and DSL for high-speed Internet access, and deliver citywide and even nationwide highspeed Internet access. For fixed applications such as cellular backhaul, WiMAX almost assuredly has a great future. However, its ability to reach broad markets rests primarily on its ability, yet unproven, to work in a vehicular environment—e.g., driving at 70 mph down the New Jersey Turnpike (see the sidebar, "Will WiMAX Hit the Road?"). Freescale Semiconductor has been developing RF power technology geared to WiMAX infrastructure applications for many years, and in 2005 released its first products.

More than even the most stringent digital cellular modulation schemes, WiMAX demands greater linearity and efficiency from RF power devices, thanks to the 64-state quadrature amplitude modulation (64QAM) and orthogonal frequency-division multiplexing (OFDM) that make it so appealing in delivering higher data rates and greater network capacity.

Achieving the performance of an RF power device for WiMAX service requires far more than simply meeting a given output power. Rather, the device must produce the desired output, while also delivering high efficiency and the unprecedented linearity that 64QAM and OFDM demand. To achieve optimum linearity, RF power devices are generally operated at power levels less than the maximum rating. As a result, a RF power transistor destined for WiMAX service must meet the contradictory goals of achieving the necessary linearity and efficiency, while also delivering the required output power. Complicating things, "peak" power, while an acceptable benchmark for many applications, is about five times higher than the "average" power specified for WiMAX systems.

The company has long offered Laterally-Diffused Metal Oxide Semiconductor (LDMOS) RF power transistors that deliver the performance needed for operation in infrastructure of 2.5-GHz WiMAX systems. However, the company's new MRF7 S38010H, MRF7S38040H, and MRF7S38075H LDMOS devices (see figure) extend this capability to the 3.5-GHz WiMAX band that will be used in Europe and other countries—the first LDMOS devices to achieve this goal. They're fabricated in Freescale's seventh-generation high-voltage LDMOS (HV7) process, operate at +28 VDC, are internally matched, and are enclosed in Freescale's advanced low-thermal-resistance packages that incorporate electrostaticdischarge (ESD) protection.

The peak power ratings of the new Freescale WiMAX devices are 10, 40, and 75 W (see table), which results in an average power rating of to 2, 8, and 16 W in the 3.4-to-3.8-GHz range (when tested using a 7-MHz-wide WiMAX IEEE 802.16 signal). The alternatives to LDMOS (and other silicon technologies) are GaAs and GaN, which while delivering exceptional performance produce devices that are significantly more expensive. Employing LDMOS devices at 2.5 and 3.5 GHz can produce a dramatic reduction in the cost of a WiMAX base station—as much as 30 to 80 percent, making them less expensive than any other type of compound semiconductor device designed for 3.5 GHz WiMAX service.

The roadmap for LDMOS in high-frequency RF applications is a bright one, with future frequency extensions higher than 3.8 GHz a virtual certainty. Anyone who doubts this should reflect on the fact a decade ago, the general consensus was that LDMOS was a wonderful technology for applications to about 1 GHz. Today at 3.8 GHz LDMOS delivers even better overall performance than it did a decade earlier at 1 GHz. The new WiMAX power transistors are the next first step on a long road of increased capability.

Freescale Applies Cellular Know-How To Refine Performance in Industrial Applications
The importance of RF power in wireless applications is easy to see, even for those without a technical background. However, powering systems in hundreds of Industrial, Scientific, and Medical (ISM) applications are millions of RF power transistors that do their jobs with little recognition. But without them, their host products would be worthless.

The ISM bands were created for non-commercial applications, and are largely unregulated by organizations such as the FCC throughout the world. These applications are complemented by others operating at much lower frequencies (down to near DC) that do not fall under the ISM umbrella, as defined by international regulatory bodies. Unlike communications applications such as cellular telephony, ISM applications are extremely diverse, ranging from garage door openers to laser exciters, plasma generators, magnetic-resonance-imaging (MRI) and medical-telemetry systems, RF heat sealing, and dozens more (see the sidebar, "Defining ISM").

There have been few dramatic performance enhancements in RF power devices for these applications over the last decade. As a result, the devices available five years ago are recognizable today, and pretty much unchanged. Freescale entered the ISM marketplace for this very reason. The company's intense development of high-power LDMOS RF power transistors for cellular base stations has produced major improvements in every key performance parameter. Freescale's over molded and air-cavity packages contribute greatly to reliability, manufacturability, and low cost as well.

The new RF transistors include three 50-V parts that operate to 450 MHz and three 28-V models that operate in the 2.45-GHz ISM band. The 450-MHz devices are designed using Freescale's VHV6 process and are housed in Freescale's award-winning lowthermalresistance plastic packages. The MRF6V2300NB (see figure) is Freescale's flagship ISM product, and delivers 300 W peak output power (single-ended), with typical efficiency of 68 percent and gain of 27 dB at 220 MHz— higher efficiency and gain than any comparable RF power transistor operating at this frequency.

The device's high gain reduces the number of gain stages required to deliver a given output power, which results in lower parts count and lower system cost. The two other 450-MHz devices include the 150-W MRF6V2150NB (69-percent efficiency and 25.5-dB gain), and the 10-W MRF6V2010NB driver (68-percent efficiency and 25-dB gain). The 2450-MHz devices include the 140-W MRF6S24140H (ceramic package), the 190-W MRF6P24190H (ceramic package), and the 20-W MW6IC2420NB driver (plastic package) (see table).

The VHV6 RF LDMOS (very high voltage, sixth-generation, RF laterally diffused metal oxide semiconductor) process is a 50-VDC enhancement to Freescale's widely accepted 28-VDC LDMOS technology. The increase to a 50-VDC supply voltage yields higher power levels and attains performance that exceed those available today in the ISM marketplace.

Higher gain and high output power mean that fewer finalstage transistors and fewer stages are needed to produce the power output a specific application requires. Gain stages, which in addition to driving up power consumption, increase the size of the power amplifier, add circuit complexity, and increase parts count, cooling overhead, and overall system cost.

For a 1.2-kW, 450-MHz RF power amplifier, only two gain stages are required—eight MRF6V2150Ns driven by a single MRF6V2010N, resulting in more than 50 dB of gain. This is a significant advantage for designers of many types of systems, for whom lower power consumption, less board real estate and parts count, and fewer concerns over dissipation of heat generated by the amplifier, are of paramount importance. Freescale's new RF power devices for the many ISM applications are sampling today, production devices will be available by the end of the year.