Microwave Engineering Europe - November 2008 - (Page 29) RF SWITCHES 29 CMOS-On-Sapphire RF switches enable multimedia handsets By Dylan J. Kelly, Peregrine Semiconductor Corporation ortable multimedia devices must handle a growing number of frequency bands. As a result, most current mobile handsets include numerous high-selectivity filters. Since each handset generation has supported more frequencies, the number of necessary filters has grown, and designers have added switching elements in order to manage the filters and optimize the link budget. These switching elements have unique requirements. For instance, they need to be available in high volumes, offer high linearity, and be easy to integrate into existing design flows. And, as with other components in the consumer electronics market, they must be low cost. Finally, because they are used in handsets, the form factor, bill of materials, and footprint are of utmost importance. Fortunately, CMOS on sapphire RF switches are already available in singlepole, nine-throw (SP9T) configurations, making it possible to minimize the number of switching elements (which reduces footprint and eases design) and maximize linearity. Evolving needs The frequency bands used for cellular communications vary around the world. To support a worldwide handset design, then, handset components need to provide multi-band support. A typical GSM phone might support up to four bands (850/900/1800/1900 MHz). The likely successor to GSM is UMTS, and it will also require multi-band support to enable global roaming and data transmission [1]. When this multi-band requirement is combined with the need for high integration, low power, small size, and maximized link budget (high linearity), then the task facing systems designers becomes even more challenging. Cellular phone service now reaches about 80 percent of the world’s population, with nearly 3 billion worldwide subscribers. In the early days of cellular services the handsets were single band, and designers used PIN diodes for switching due to their high performance and low cost. However, because they require long quarter-wave transmission lines and large forward bias currents to P Figure 1: A multiband handset system layout is simplified with an UltraCMOS SP9T switch. operate, PIN diodes failed to meet the size and performance requirements of quad-band GSM. Even as these new bands were added to offer new services and reach new markets, service providers continued to scrutinize performance and demand better range for more effective network operation. To satisfy the high linearity required to meet these latest link budget requirements, handset designers typically use high selectivity filters in the system front end instead of the PIN diode. Multiband leads to new materials The four-band requirement of GSM required handset designers to look for new materials for the switching elements. They considered IC-based solutions manufactured using UltraCMOS™ or GaAs. These technologies solved the numerous implementation problems with PIN diodes for multi-band operation, and, quickly replaced PIN diodes for the switching function. The challenges, however, had only just begun with four-band GSM. In fact, as designs are developed for LTE handsets, the demand for more switching continues to grow at an almost alarming speed. For instance, in 2007, the majority of 3G handsets shipped with quad-band GSM/EDGE and single-band WCDMA, which requires a SP7T switch. By 2008, the integration of three WCDMA bands was required, increasing the switch complexity to a SP9T (Figure 1). This trend shows no indication of slowing down. Currently, there are nine WCDMA bands around the world, and that number is expected to climb. This will increase the need for front end switching. In terms of materials, handset designers are effectively faced with two choices in switches: UltraCMOS or GaAs. But which offers the best balance of performance, size, and cost? RF performance Since the RF front-end switch must support multiple signals simultaneously, it needs to be the most linear element in the handset in order to ensure the expected range. In addition to linearity, some additional considerations include: voltage handling, die footprint, system size, ease of design, and ESD tolerance. Microwave Engineering ● November 2008 ● www.mwee.com http://www.mwee.com
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