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Maximize multiband handset performance

Posted: 03 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:antenna access? RF signal path? handset design?

The RF signal path in a handset has become increasingly crowded. Cellphones have moved rapidly from dual-band to tri-band to even quad-band. These complex phones also need to handle various signals for peripheral radios such as Bluetooth, Wi-Fi and GPS. This complexity will grow even more as WiMAX and Long-Term Evolution are added. In a handset, the antenna switch controls antenna access for all of these radio signals, essentially acting as the gatekeeper.

Multiband handset design is challenging because all these signals operate on different bandwidths, but they all need access to the antenna. To achieve optimal performance and footprint, it is better if they can access the antenna through a single RF switch. For switch manufacturers, this has meant a corresponding evolution from single-pole four-throw (SP4T) to SP7T, and now, SP9T configurations to handle the increased number of signals.

Manage signal traffic
The 3G handset market has migrated to W-CDMA to support Internet, multimedia and video. In response, GSM evolved into a dual GSM/W-CDMA technology. As a result, handset complexity has reached unprecedented levels.

Designers of the RF front end are responsible for the antenna switch module (ASM), front-end module (FEM) and the transmit module. For RF designers, a multiband scenario means significant architectural, performance and cost challenges. Any design trade-off in a multiband phone requires the handset to meet or exceed the performance levels of all the standards handled.

Typically, a multimode, multiband mobile handset uses a single power amplifier (PA) module to handle the quad-band GSM/Edge signals. On the other hand, each W-CDMA band requires its individual PA. As a result, a quad-band GSM phone with one W-CDMA band needs at least a single-pole, six-throw (SP6T) switch to manage all of the signal paths. Alternatively, designers can use a diplexer and two SP3Ts (a popular GaAs configuration), but this results in higher insertion loss than when using a single SP6T switch.

RF designers need to keep a close eye on insertion loss because it directly impacts the effective power-added efficiency (PAE) of the PA. GSM PAs are typically run in saturation at up to 3W, with an average PAE of 55 percent. This level of efficiency is necessary to ensure battery life, since half of the total handset current drain is from the PA. Designers need to maintain the PA's PAE a high priority.

Some of the earliest multiband GSM/W-CDMA handsets included separate signal chains for W-CDMA and GSM, with a separate antenna and radio design. While this worked for prototypes and first-generation designs, market pressures required a more cost-effective, space-saving approach. Clearly, the industry needs integrated ASMs that handle seven or even nine signals.

In response to this need, SP7T switches were developed to support handset architecture with one W-CDMA and four GSM bands. The PE42672, for instance, is a monolithic SP7T developed on UltraCMOS process technology, which delivers a third-order intercept point (IP3) of +68dBm, a measure of linearity performance that enables 3GPP IMD3 specification-compliant handset designs and efficient RF front ends.

The SP9T switch can be configured to handle multiple bands of W-CDMA, GSM and peripheral radios. For example, the switch shown in the figure is handling three bands of W-CDMA, with paths to duplexers and three PA modules (each W-CDMA band requires its own PA and duplexer). The device also handles quad-band GSM/Edge, which has a single PA module associated with it (which contains two PA ICs). In effect, this device has to route five high-power signals through a single switch that is controlled by a simple decoder.

As multiband architectures have grown, so has the requirement for the number of PAs and associated filters. In essence, the technical demand on the PAs has not changed, but handset designs need to use more of them. What has changed is the need for an extremely efficient method to route all of the RF signals to the antennathe monolithic switch.

Adding more bands to the handset has greatly increased the technical requirements of the switch, and the linearity and harmonic requirements of W-CDMA have put a large strain on the device's performance. For instance, a switch is now generally agreed to need an IP3 of better than +65dBm. In previous GSM-only designs, there was no comparable linearity requirement. Not only is +65dBm a new requirement, it is a difficult one for switch manufacturers to achieve.

Keep it small
As the popularity of multiband handsets grows, the need for highly integrated, small antenna switches becomes more pressing. In terms of footprint, a GaAs SP7T measures 1.6mm x 1.5mm, while a comparable SP7T switch design using 0.5?m silicon-on-sapphire with equal or better small- and large-signal performance measures 1.2mm x 1mm, 50 percent smaller. The GaAs E/D pHemt or J-pHemt SP9T switches measure 1.9mm x 1.5mm.

Another way to shrink footprint is to flip-chip mount the switch to a low temperature co-fired ceramic substrate without underfill, eliminating the area previously required for wire bonding.

By employing switches made using UltraCMOS, designers can eliminate the decoder, blocking capacitors and the diplexer that are required with other switch technologies. Combined with chip-scale packaging technology, this process dramatically reduces the size and thickness of ASMs. In addition, inherent ESD tolerance and a monolithic CMOS interface simplify implementation and use. Finally, the high yield of UltraCMOS processes and scalability to additional switch throws provides a roadmap to even higher levels of integration for future generations of handsets, promising to answer the continuing challenge of shrinking real estate in the multiband handset.

Impact on components
The technical requirements of the multimode, multiband GSM/W-CDMA handset have overcome the limits of traditional RF IC technologies such as GaAs. Most critically affected by these ultra-high performance specs are the antenna and the RF switch.

This SP9T is handling three bands of W-CDMA, with paths to duplexers and three PA modules.

It is important to recognize the significant impact on the system antenna. It must effectively radiate from 800MHz to 2,200MHz, a daunting task given the footprint allowed for the tiny antenna. New techniques are now being looked at to address this issueconsidering antenna matching issues, using switches and lumped tuning elements. Ultimately, it is the RF switch that must be capable of switching up to nine paths of high-power RF signals with low insertion loss, high isolation and exceptional linearity.

The advancement of new manufacturing processes and highly-integrated designs are making it possible to realize the necessary multiband performance in the latest, most complex and smallest portable devices.

- Rodd Novak
VP of Marketing
Peregrine Semiconductor Corp.

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