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Integrate RF signal chains with CMOS

Posted: 16 Feb 2007 ?? ?Print Version ?Bookmark and Share


By Rodd Novak
Peregrine Semiconductor Corp.

Consumers have grown to expect increased functionality and convenience from their mobile handsets and PDAs. As a result, these products are rapidly evolving from voice and data communications into a mobile environment that contains enjoyable features such as audio libraries, video playback, interactive games, digital cameras and mobile TV. In addition to the multiple mobility standards for cellular service, today's handsets commonly support WLAN, UWB, Bluetooth, GPS and DTV. Diverse technologies are required to enable these features, requiring multiple radios and application software packages that run simultaneously while taking into account the existence of these other services.

One device
Portable device designers face challenges to keep up with the demand as consumers take for granted their ability to download email, receive location-based information, talk on a wireless headset, view live broadcast TV, download movies, and send video and images on a single device. In addition, designers must chart a course to be able to integrate future broadband services into a single mobile device.

In 3G, mobile handsets moved beyond voice transmissions to become multimedia devices. With 4G, multiple technologies are supported by highly integrated circuitry within the handset. The challenge for portable system designers is to bring together mobile communications, computer networking, personal area networking, and broadcast into a single system. A 4G device will not only take into account existing mobility standards including GSM, GPRS, EDGE, UMTS, W-CDMA and HSDPA, but it also will support data rates of 100 Mbits/s to 1 Gbit/s and could feature an Internet Protocol (IP) core, OFDMA modulation, and MIMO antenna technology, as well as support for VoIP/V2IP and mesh networking.

Technology challenges and solutions
Different technology challenges face handset designers and their suppliers in the race to integrate the RF signal chain. For handset designers, the challenge is finding highly integrated, low-power components. For their suppliers, the challenge is to find the right process technology and to leverage engineering expertise in order to shrink device sizes while packing more RF circuitry onto a die. Overcoming these challenges requires a smart design that is integrated, cost-effective, and crafted with the entire signal chain in mind. An effective technology to satisfy all of these needs, including high-power applications, is CMOS.

CMOS is the dominant technology for monolithic integration, and it has driven rapid advancements throughout the communications revolution. Always the leading technology for digital baseband processing, CMOS is regularly used for analog devices, enabling new transceiver architectures and consistently used in analog-to-digital converters (ADCs), phase locked loops (PLLs), filters, and in-phase/quadrature (I/Q) demodulators. The technology is also now making inroads into RF and microwave devices. To enable high power applications, the key is selecting a suitable CMOS process technology. UltraCMOS, a silicon on insulator (SOI) technology, meets this need.

High-power CMOS process technology
UltraCMOS uses SOI technology that deposits a very thin layer of silicon on an insulated sapphire substrate. Like CMOS, UltraCMOS technology offers low power, manufacturability, repeatability, and scalability benefits in an easy-to-use process that allows the re-use of intellectual property blocks and the highest levels of integration.

Unlike CMOS, UltraCMOS provides equal or better performance as compared to GaAs or SiGe for mobile, RF, and microwave applications. UltraCMOS and pHEMT GaAs both offer the same levels of small-signal performance and have a similar net ON-resistance. In addition, UltraCMOS features onboard decoder/drivers while delivering superior linearity and electrostatic discharge (ESD) performance to GaAs or SiGe.

For highly complex applications such as the latest multimode, multi-band phones, choosing the right process technology becomes critical. For example, in these applications, the antenna must cover 800-2,200MHz and the switch must be able to manage up to eight paths of high-power RF signals with low insertion loss, high isolation, superb linearity, and low power. Correct selection of the process technology can improve the availability of technical options, in turn improving the performance of antennas and RF switches, thus enhancing performance of the device overall. Importantly, when engineers use a single process technology across the entire design, the likelihood of higher integration improves.

The latest developments in UltraCMOS RFICs are SP6T and SP7T antenna switches. These 3GPP-compliant switches satisfy W-CDMA and GSM requirements, allowing designers to use a single radio in specification-compliant W-CDMA/GSM handsets, and still achieve industry-leading performance. SP6T and SP7T antenna switches utilizing Peregrine's HaRP technology enhancements demonstrate second harmonics of -85dBc; third harmonics of -83dBc; and third-order intermodulation distortion (IMD3) of -111dBm at 2.14GHz (Figure 1).

Figure 1: IMD3 performance of HaRP-enhanced switches exceeds 3GPP industry standards.

Linearity standards from 3GPP are set high, with an IP3 of 65dBm. Typical switches in competing technologies demonstrate an IP3 of approximately 57dBm. UltraCMOS SP7T switches offer an impressive IP3 of 68dBm, offering improved linearity over other choices and exceeding 3GPP standards. For antenna ESD tolerance, other technologies typically provide a 0.5kV tolerance. UltraCMOS SP7T switches deliver 4kV ESD tolerance.

Integrate the RF chain
Today's handset designers must support an unprecedented number of applications in a single small device, so the need for more integration may never have been so great. Integration can take many forms, but it is especially advantageous when used to reduce the number of passive components in a system. The circuitry in mobile handsets, regardless of the interface standard, consists of approximately 75 to 85 percent passive components, such as capacitors, inductors, and resistors. For example, the Nokia 3300 music phone includes a total of 406 components, and 355 of them are passives. Reducing the number of passive components is an important factor in managing new designs.

One way to reduce RF component count is to use integrated passive technologies, such as low-temperature co-fired ceramic (LTCC). Unfortunately, because of interconnection and test costs, these integrated passives have only been proven effective when implemented at the module level. While modules can be cost-effective when compared to discrete implementations, they typically require the use of multiple suppliers and extensive testing.

LTCC modules have been used with some success for antenna-switch modules (ASMs) in GSM handsets. (ASMs are responsible for the front-end RF signal routing and harmonic filtering for the power amplifier). And, most early implementations of high-volume dual- and triple- band GSM phones used PIN diodes combined with passive components integrated into the substrate. However, during the design of quad-band GSM phones, a new trend began emerging as ASM designers began to look at more integrated technology choices, such as UltraCMOS.

Component counts in ASMs decrease by as much as 60% after using UltraCMOS SP6T and SP7T switches that include the integrated matching circuitry and harmonic filters. And, because UltraCMOS is a monolithic solution, integrated designs also eliminate the high-priced packaging and die to die interconnect issues associated with modules. Because of its highly-insulating substrate UltraCMOS can also be used to integrate high-Q passives. In addition, these UltraCMOS passives benefit from the repeatability of semiconductor processing with inductor tolerances of approximately 2% and capacitor tolerances of 5%.

Using a single process technology across the entire design improves the chances for higher integration. CMOS is already widely used in the IF and baseband sections of a mobile phone. Moving into the RF front end can hasten further integration. Figure 2 shows a multi-technology approach to an RF front end (2a) as compared to a solution implemented using an advanced UltraCMOS approach. (2b). The UltraCMOS device integrates the low-pass filters, DC/DC converter, controller, decoder, and driver as well as antenna matching circuitry, thereby requiring fewer components than a multi-technology approach.

Figure 2: Increased integration of the RF signal chain can be achieved with an UltraCMOS solution as compared to a multi-technology switching approach.

Cut power consumption
The two most power-hungry sections of mobile handset designs are the baseband processor and the RF front end. Power amplifiers (PA) draw the majority of the power in the RF front end. Keys to achieving less power consumption are to design the rest of the front end to use as little power as possible and not impede the work of the PA. In terms of available choices, a GaAs switch with decoder draws 600?A, while a PIN diode implementation consumes 10mA. In typical RF front-end applications, UltraCMOS SP7T switches consume 10?A.

Insertion loss is another factor to consider when managing power consumption. The important point here is to select front-end components that do not impede the efficiency of the PA. Typically, PAs used in GSM handsets run in saturation at up to 2W, and their power added efficiency (PAE) is generally around 60%. This level of efficiency is critical to extend the battery life of the handset. However, a high PAE will be reduced when using a front-end architecture with high insertion loss. Figure 3 shows the effective PAE and insertion loss from the PA to the antenna for four different origination PAE levels. Assuming a front-end insertion loss of 1.5dB, a PA with a 60% origination PAE would be reduced to just 42.5%.

Figure 2: Insertion loss reduces power added efficiency (PAE) of a power amplifier.

Designers must continue to shrink component sizes, improve functionality, minimize critical interactions, and develop cost-effective circuitry to keep handset sizes small, power consumption low, and performance on target for 4G applications. As we look to the future of portable communications devices, it is time to consider new process technologies in order to achieve tomorrow's goals. Fortunately, UltraCMOS offers designers the comprehensive performance required to meet demanding specifications as well as exceptional linearity, ESD tolerance, and integration potential.

About the author
Rodd Novak
is VP of Marketing and directs the company's strategic business development activities. Comments may be sent to

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