2.5 V GaAs pHEMT Switches for GSM Handsets

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2.5 V GaAs pHEMT Switches for GSM Handsets

M/A-COM
Lowell, MA

A new line of robust GaAs pHEMT switches for operation in 2.5 V GSM handsets has been developed. These switches feature performance that is unrivaled in the industry for harmonics at 2.5 V in addition to offering low insertion loss, high isolation, low current, small size and low cost. The key to these performance levels has been to leverage the company's advantages in circuit design, modeling, pHEMT processing and miniaturized packaging into cost-effective products that are shipping in high volume to meet the demands of current and next generation cellular phones.

These products have superior harmonic rejection versus control voltage characteristics. Unlike other competing products which nominally work closer to 3 V, these switches offer harmonic knee voltages that are substantially lower than 2.5 V, even reaching down as low as 2.0 V typical. A lower nominal knee voltage allows the switch to maintain its superior harmonic performance for higher yield against voltage, temperature and process variations. It is this performance, combined with its inherent compact size and low current, that gives integrated GaAs switches the advantage over existing PIN-based module solutions for emerging multi-mode, multi-band 3G phones.

A number of topologies and configurations have been developed, ranging from symmetric SP2T and SP3T architectures to more complicated SP2T/SP4T modules that include embedded CMOS drivers. Products are available in both packaged and die form to serve both discrete and higher level module handset solutions.

Overview

GaAs switches have been used for cellular handsets for many years for both high power and low power RF signal routing applications. The combination of low loss, high isolation, low current, small size and low cost have made these components a popular choice for a vast number of system designs.

In the past GSM handsets have typically used either 4.5 V GaAs switches or PIN diode-based modules as solutions for front-end antenna switching. However, market forces for reduced size and cost, as well as the introduction of emerging multi-mode, multi-band handsets are forcing designers to innovate new solutions.

In the case of 4.5 V GaAs switches, a reduction in parts count and board size can be gained if the 4.5 V operating voltage can be reduced to 2.5 V nominal. This is accomplished by simply eliminating the boost circuitry required to generate the 4.5 V supply voltage. Regarding PIN-based modules, the addition of WCDMA functionality to existing dual-band phone architectures makes the module size, complexity and quiescent current draw unattractive.

The drive for smaller size and reduced parts count at the system level is now forcing the integration of previously discrete components. For example, a variety of front-end modules for GSM are now emerging for 2.5G & 3G phones that integrate the switch, diplexer and receive SAW filters into a single compact low cost assembly.

2.5 V Switch Solution

The key to meeting the needs of these advanced modules as well as more discrete phone solutions has been the introduction of GaAs power switches that operate at 2.5 V with acceptable margins for voltage, temperature and process variation. Figure 1 provides definitions for a switch's harmonic rejection versus control voltage characteristic. The nominal knee voltage, Vk, needs to be low enough so that natural excursions seen in the environment and across unit-to-unit production allow the component to meet the overall system requirements.

Several years of research and development, including a careful study and modeling of the factors which influence harmonics at low voltages, was recently completed. The outcome of this work was the implementation of several proprietary and patented innovations in the area of design, fabrication and test for low harmonic switch products. A family of GaAs switches has been successfully developed for the GSM handset market with high harmonic rejection at low control voltage while maintaining optimum insertion loss and isolation performance.

Table 1 summarizes existing products that are now offered to serve the 2.5 V GSM handset market. These are offered in both plastic packaged and die form to serve both integrated and discrete handset solutions.

Symmetric Switches

One class of switches is the single-pole, multi-throw switch - SP2T (single-pole, two-throw), SP3T and SP4T products that have symmetric, or balanced RF performance at each port. An example is the MASWSS0020 SP4T that exhibits harmonic rejection of greater than -70 dBc with a control voltage of less than 2.2 V when injected with a +34 dBm 1 GHz signal. The results are presented in Figure 2 . The insertion loss is 0.55 dB at 1 GHz and 0.8 dB at 2 GHz, and the isolation is 29 dB at 1 GHz and 22.5 dB at 2 GHz. It is available in an industry standard FQFP-N 16-lead package that measures 4 mm x 4 mm x 1.0 mm (max). This package is leadless and features an exposed backside ground plane metallization that provides superior thermal and RF performance at frequencies well beyond 2 GHz.

The SW-489 is a SP3T that has 1 GHz insertion loss of 0.45 dB and is offered in a 3 mm x 3 mm x 1 mm (max) FQFP-N 12-lead package. This same functionality can also be found in the MASWSSS0033, which has a newer, thinner 3 mm x 3 mm x 0.8 mm (max) package. The MASWSS0006 SP2T has 0.35 dB insertion loss at 1 GHz and is housed in a miniature SOT-26 package.

Triple-band and 3G Switch Modules

A family of GSM switches with integrated CMOS drivers to support emerging 2.5G and 3G handset architectures has also been developed. A prime example is the SW-444 SP5T product that has been developed for triple-band phones. It features a common antenna port that connects to GSM transmit, GSM receive, DCS/PCS transmit, DCS receive and PCS receive functions. The RF performance of this switch has been optimized to meet the requirements of each port.

The inclusion of an integrated CMOS driver allows for simplified logic control. This driver enables use of control voltages as low as 1.8 V by incorporating a circuit that raises the input signal levels to the system supply level, V_dd , which typically runs at 2.7 V. Figure 3 shows harmonic rejection versus Vdd for the SW-444 GTX path. It has an outstanding harmonic rejection at a system voltage of less than 2 V.

Other related 3G module products that integrate a GaAs switch with a CMOS driver include the SW-488 SP2T/SP3T and the MASWSS0017 SP2T/SP4T, both of which support dual-mode GSM and WCDMA/ UMTS functionality. All of these designs feature standardized leadless FQFP-N packaging for maximum RF performance in a minimum board footprint.

Conclusion

A family of robust 2.5 V switches has been developed to serve the next generation GSM handset market. They have been optimized to not only work down to 2.5 V, but to do so comfortably with sufficient margin for process and temperature variation. These designs are offered in a wide range of topologies and are available in plastic package and die form.

M/A-COM, Lowell, MA (800) 366-2266, www.macom.com.
Circle No. 302