Integrating RF Signal Chains with CMOS

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, WCDMA, 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 to 2200 MHz 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 WCDMA and GSM requirements, allowing designers to use a single radio in specification-compliant WCDMA/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 -111 dBm at 2.14 GHz (Figure 1).

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