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 environmente.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 countriesthe 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 stationas 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 requiredeight 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.
Removing The Burden Of RF Integration From Cellular System Builders
Over the years, design-engineering teams tasked with creating cellular phones have warily regarded the transceiver portion as unfamiliar territory, an island unto itself. While of necessity, the RF and digital designers at phone manufacturers have grown a bit closer, the less time digital designers spend dealing with the vagaries 'fields and waves' the better. Complicating matters, when transceiver manufacturers make changes to their designs, regardless of how minor, designers at the phone manufacturer have typically been required to adjust a large number of variables to accommodate the changes, even though the basic functionality transmit and receiveremains the same. What's more, calibration of phones in production requires a considerable amount of time, since many variables must be addressed. This situation hasn't changed much, until now.
Freescale has enhanced its MMM6000 EDGE transceiver (part of the RFX family of frontend subsystems) to significantly reduce the burden on the phone manufacturer when integrating an EDGE transceiver and when testing phones on the production floor. Taken together, these enhancements have shortened the design process and reduced time spent per phone in production test by an order of magnitude.
Freescale realized that for the benefit of phone manufacturers, RF transceivers should be as integrated as possible with the digital circuits of the phone. Their achievements can best be shown by illustrating two examples that in the past have required significant effort by phone manufacturers.
In a frequency synthesizer, the synthesizer's integer divide ratio, the numerator and denominator of the factional divide, and other parameters are set by software. The specific details of this programming depend on the circuit and architecture. Freescale's MMM6000 devices (see figure) solve this problem by removing as many of the details from the software interface as possible. In the synthesizer case, the software simply commands the RF IC to (for example) "go to channel 1." Since the device has been programmed with all the details of that channel (as specified in the 3GPP standard), no more programming is necessary. This allows Freescale to freely incorporate circuit and architectural improvements to the synthesizer (or other components) without placing the burden on the software team to learn how to implement them. This is direct contrast to the usual process in which designers have to program every component of every channel, a tedious process that the Freescale approach simply eliminates.
Automatic gain control (AGC), a basic function of every second-and third- generation phone, is another good example. Setting the gain on each of the amplifiers in a transceiver takes programming time. With Freescale's approach, when an AGC state is defined, not just one but all amplifiers are automatically set to the proper gain setting. While this may seem a minor improvement, it has already proven to significantly reduce development time, and allows designers to migrate from one device generation to another with far less difficulty.
Freescale has also focused on eliminating as many calibration steps as possible during production test, eliminating adjustments that have historically been calibrated in the phone factory and performing them automatically on the chip. Phone factory calibration for RF parameters with typical RF solutions can take 200 to 300 s to accommodate many different power levels and to optimize EDGE performance.
Freescale's architecture requires calibration at only two power levels, which takes less than 20 s, an order of magnitude reduction that can dramatically increase phone throughput on the production floor. The result is that the number of phones a given production line can handle during a single shift can, conservatively, be doubled.
Both of Freescale's enhancementsreduced design cycle time and the time spent in production testare accomplished by integrating digital functionality into a traditionally analog subsystem. This "intelligence" allows a single command to facilitate multiple actions, all of which must be performed manually with "human intervention" in competitive products. Moreover, RF architectures typically require a designer involved with the physical layer (Layer 1) of the phone to understand precisely what is occurring at every point in the RF path and the transceiver's architecture, and orchestrate the required functions from the RF device level straight through to the antenna. The Freescale architecture views the RF section as just another hardware peripheral that must interface with the microcontroller or DSP at the baseband level. Layer 1 is programmed as a whole, rather than in its many discrete elements.
The high efficiency and exceptional linearity required by GSM/EDGE and CDMA, that employ complex modulation schemes such as QPSK, DQPSK, and HPSK have driven many RF transceiver vendors to use some form of polar modulation. The MMM6000 transceiver employs a revolutionary small signal polar modulation approach Freescale calls the "Polar Plus" transmit architeture (see the sidebar, "The Polar Plus Advantage.")
Compared to other polar modlation formats, Polar Plus modulation offers a significant advantages in performance and circuit design, while making it easier to design the power amplifier into a phone or other wireless-enabled product. Combined with time and cost-saving advantages of reducing calibration time and easing RF integration into a largely digital product, the MMM6000 product stands out as a optimal choice for designers of cellular-enabled products.
Freescale Puts ZigBee Specification Together
The buzz about the ZigBee specification is getting louder and louder as the technology, introduced in 1998, gains the momentum it needs to permeate home, business, and industrial applications. While there have been attempts to develop a wireless standard to serve the almost limitless control and sensing markets for more than a quarter century, the ZigBee specification appears set to take flight. The Zig Bee Alliance (www.zigbee.org) has more than 200 member companies, and the necessary components, development tools, and control software required to put the ZigBee specification to use are now available. The key to developing a ZigBee network goes beyond the hardware itself to reference designs, development tools, and other support tools that speed product development As one of the first companies to embrace this technology (the company's physical layer was chosen as the basis of the standard), Freescale already has a comprehensive ZigBee solution, from RF chip sets, to microcontroller units (MCUs), sensors, reference designs, protocol stack software, and development tools.
The latest enhancement to Freescale's ZigBee portfolio is the BeeKit wireless connectivity tool kit, which includes a simple interface and framework to allow developers to configure point-topoint, IEEE 802.15.4, and ZigBee applications. The Beekit tool includes the company's BeeStack? protocol stack and configuration tools that work seamlessly with the company's platforms that are compliant with the ZigBee specification. It's compliant with the next generation of the ZigBee Alliance's home control protocol stack, which is expected to include a home automation profile. BeeKit also supports Freescale's IEEE 802.15.4-compliant stack and Simple MAC (SMAC) software. After a specific design has been completed, the application is exported to Freescale's CodeWarrior integrated development environment where it can be debugged.
In addition to the BeeKit tool, Freescale provides one of the industry's most comprehensive arrays of ZigBee specification ready components. First and foremost are the transceivers that form the core of ZigBee networks. The company's MC1319x transceiver family is well suited for Zig-Bee and 802.15.4 applications, incorporating a dual-data modem and a digital core to help reduce MCU processing power requirements and execution cycle time.
Virtually any MCU in the Freescale family can be used, thanks to the serial peripheral interface (SPI) connecting the RF IC and MCU.
The MC1320x is the company's next generation of 802.15.4-compliant transceivers, and includes an integrated transmit/receive (T/R) switch to help reduce the need for external components, which in turn reduces the bill of materials and overall system cost. The transceivers support Freescale's software stack options, the Simple MAC (SMAC), 802.15.4 MAC, and full ZigBee stack. Heading the list of the components is the MC1321x System in Package (SiP), which integrates a MC9S08GT MCU and a MC1320x transceiver in a single 9 9-mm LGA package (see figure) . Flash memory can be selected from 16 to 60 kbytes. The MC13211 provides 16 kbytes of flash memory and 1 kbyte of RAM, and is well suited for cost-effective proprietary applications in point-to-point or star networks employing SMAC software. For networking on a larger scale, the MC13212 (with 32 kbytes of memory and 2 kbytes of RAM memory), and IEEE 802.15.4 MAC.
In addition, the MC13213 (60 kbytes of memory and 4 kbytes of RAM) and the ZigBee protocol stack are designed to help engineers produce fully certifiable Zig-Bee products. The MC13213 provides comprehensive encoding and decoding, programmable clock out for the baseband MCU, standard four-wire SPI operating at 4 MHz or higher, extendedrange capability with an external low-noise amplifier and power amplifier (PA), and programmable output power.
Completing the package are Freescale's MCUs, which interface with the MC1319X and 20x transceiver families and run the 802.15.4 or ZigBee application. Freescale offers the industry's most comprehensive family of microcontrollers and development tools, which suit just about any application. The HCS08 is a family of low-voltage, low-power microcontrollers (designed for use with the MC1319x and MC1320X families) 8-b MCUs that incorporate features to extend battery life and integrate peripheral and memory combinations. At the highest level of performance, Freescale ColdFire ® processors, HCS12 16-b MCUs, i.MX applications processors, 56800/E hybrid controllers, and PowerQUICCR integrated communications processors containing PowerPC ® cores are engineered to deliver the desired performance.
Sensors also are key elements of ZigBee installations. If the application employs wire-line control networks, Freescale's acceleration and pressure sensors as well as ion and photo smoke ICs fit seamlessly. They are based on microelectromechanical systems (MEMS) technology and use standard OEM hardware interfaces.
The company's IEEE 802.15.4 MAC software is standards-compliant and smaller than Bluetooth-TM wireless technology because the IEEE 802.15.4 standard needs small amounts of on-chip memory and very little microcontroller processing power (see table). It supports peer-to-peer, star, and mesh network topologies as well as upper ZigBee layers, power-saving modes (doze, hibernate, and user-configurable), an optional super-frame structure with beacons, and a guaranteedtimeslot (GTS) mechanism.
When Freescale embarked on its development path, the company realized that the extensive number of applications in which 802.15 or ZigBee could be deployed would make it essential that a designer have an array of choices in hardware and software. Beyond this, system builders would need as many resources as possible to ensure that the ZigBee networks they construct satisfactorily served the needs of customer in diverse markets. The result is Freescale's large and growing family of ZigBee-related products and services.