During the last few years, this RF signal path in a mobile handset has become increasingly crowded. Cellular phones have moved rapidly from dual-band, to tri-band, to even quad-band. In addition, these complex phones also need to handle a variety of signals for peripheral radios, such as Bluetooth, WiFi, and GPS. This complexity will grow even more as WiMAX and LTE (4G) are added. In a mobile handset, the antenna switch controls antenna access for all of these radio signals, essentially acting as the gatekeeper.Switch RequirementsMulti-band handset design is challenging because all of these signals operate on different bandwidths, yet 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, to, now SP9T configurations in order to handle the increased number of signals. These advanced switches are needed to handle the influx of additional mobile communications bands brought about by wideband CDMA (WCDMA) as well as low-power I/O radios.
We can expect handset complexity to grow and require the handling of even more bands. Most likely, the market will standardize on at least seven bands, with room for an eighth (for LTE). Even if consolidation occurs, any relief from that in the RF circuitry will quickly be overshadowed by the increased popularity of peripheral radios and functions that also need access to the antenna.
Manage Signal Traffic
The 3G mobile handset market has migrated to WCDMA in order to support Internet, multimedia, and video. In response, GSM evolved into a dual GSM/WCDMA technology. In order to satisfy global needs, current GSM phones can have up to four transmit (Tx) and four receive (Rx) paths. Adding WCDMA requires another Tx/Rx path for each new band. Current mobile handset designs are tending towards 4xGSM (850, 900, 1800, 1900 MHz) and 3xWCDMA (850, 1900, 2100 MHz) front-ends. 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. (An ASM typically includes a switch, decoder, PA low pass filters, ESD circuitry, and a voltage generator.) For RF designers, a multi-band scenario means significant architectural, performance, and cost challenges. Any design trade offs in a multi-band phone require the handset to meet or exceed the performance levels of all the standards handled.
Typically, a multi-mode, multi-band mobile handset uses a single power amplifier (PA) module to handle the quad-band GSM/EDGE signals. On the other hand, each WCDMA band requires its own individual PA. As a result, a quad-band GSM phone with one WCDMA 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. In light of this, designers need to make maintaining the PA's PAE a high priority.
Some of the earliest multi-band WCDMA/GSM handsets included separate signal chains for WCDMA 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 needed integrated ASMs that handled seven or even nine signals.
In response to this need, SP7T switches were developed to support a handset architecture with one WCDMA 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 which enables 3GPP IMD3 specification-compliant handset designs and efficient RF front ends. IP3 correlates to the devices' third-order intermodulation distortion (IMD3) performance, and these measurements over phase can be seen in Figure 1.
1. IP3 correlates to the devices' third-order intermodulation Distortion (IMD3) performance; this graph shows these measurements over phase for the UltraCMOS SP7T (PE42672) and SP9T (PE42693). The SP9T switch is one of the latest advancements in switch architectures. It can be configured to handle multiple bands of WCDMA, GSM, and peripheral radios. The switch in Figure 2, for example, is handling three bands of WCDMA, with paths to duplexers and three PA modules (each WDCMA 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.
2. This SP9T is handling three bands of WCDMA, with paths to duplexers and three PA modules.
Adding more bands to the handset have greatly increased the technical requirements of the switch, and the linearity and harmonic requirements of WCDMA have put a large strain on the performance of the device. For instance, a switch is now generally agreed to need an IP3 of better than +65 dBm. In previous GSM-only designs, there was no comparable linearity requirement. Not only is +65 dBm a new requirement, it is a difficult one for switch manufacturers to achieve. By leveraging the linearity advantages of the UltraCMOS manufacturing process, the monolithic PE42693 SP9T in Figure 2 is able to maintain the +68 dBm IP3 of its SP7T predecessor with IMD3 performance that surpasses the industry specification of -105 dBm (Figure 1).
The SP9T functionality can be realized in GaAs devices, but it will require additional components such as a CMOS decoder and driver. This will greatly affect the number of I/Os required. For instance, the SP9T in Figure 2 requires four control lines. A comparable GaAs implementation of an SP9T would require 18 control lines, making it very challenging to route in and out of a singular device. This is particularly challenging for the five high-power ports that require high linearity and isolation, because the more I/Os there are, the more likely it is for wires to couple and bond. For instance, the PCS1900 transmit band overlaps with the DCS1800 receive band. Without good isolation (35 dB or better), unwanted in-band signals can pass through the filters and desensitize the receiver.
Keep it Small
As the popularity of multi-band handsets grows, the need for highly integrated, small antenna switches becomes more pressing. UltraCMOS SP7T switches are now in volume production, with SP9T anticipated in volume in late 2007. In terms of footprint, a GaAs SP7T measures 1.6 x 1.5 mm whereas a comparable SP7T switch design using 0.5 m SOS with equal or better small- and large-signal performance measures 1.2 x 1.0 mm, 50% smaller. Currently available GaAs E/D pHemt or J-pHemt SP9T switches measure 1.9 x 1.5 mm. Compare this to an SP9T manufactured using an UltraCMOS 0.5 um process that measures 1.7 x 1.1 mm (Figure 3) and does not require off chip ESD devices and linearity enhancing matching components. The UltraCMOS roadmap anticipates that the 0.25 um version of the SP9T will measure 1.32 x 1.29 mm.
Another way to shrink footprint is to flip-chip mount the switch to a low temperature co-fired ceramic (LTCC) substrate without underfill, eliminating the area previously required for wire bonding. Currently, wafer-level chip-scale packaging is in development to produce UltraCMOS switches that can be handled like a standard surface-mount package.
By using 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 multi-band mobile handset.
Impact on RF Components
The technical requirements of the multi-mode, multi-band GSM/WCDMA handset have overcome the limits of traditional RFIC technologies such as GaAs. Most critically affected by these ultra-high performance specs are the antenna and the RF switch.
Although this article primarily addresses the antenna switch, it is important to recognize the significant impact on the system antenna. It must effectively radiate from 800 to 2200 MHz, a daunting task given the footprint allowed for the tiny antenna. New techniques are now being looked at to address this issue—taking into account antenna matching issues, utilizing switches, and lumped tuning elements. Ultimately, though, 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 as well as highly-integrated designs are making it possible to realize the necessary multi-band performance in the latest, most complex, and smallest portable devices.
About the Author
Responsible for the design and execution of Peregrine's global product marketing strategy, Rodd Novak is VP of Marketing and directs the company's strategic business development activities. He can be reached at rnovak@psemi.com.
