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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.
Multi-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.
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2. This SP9T is handling three bands of WCDMA, with paths to duplexers and three PA modules.
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