RF Antenna Switches Simplify Telematics Radios

May 17, 2006
High-frequency telematics applications require a different set of performance levels than traditional commercial RF applications in WLANs and cellular handsets.

Telematics, a combination of computer and telecommunications technologies, is being applied in several fast-growing markets, including the automotive industry. Telematics enables remote diagnostics, on-demand navigation, emergency assistance, fleet or stolen vehicle tracking, and intelligent transportation systems.

As automotive telematics devices become more widespread, designers face the same problem with the evolution of any communications application: adding features while reducing costs. While telematics can build on the experience of wireless telephony and wireless localarea networks (WLANs), the technology has its own specific needs not easily satisfied by off-the-shelf integrated circuits (ICs) designed for wireless handsets and WLAN adapters. Engineers designing high-performance, cost-effective telematics radios would prefer devices that have been optimized for the specific requirements of telematics applications.

For example, consider the functions of an RF switch in a telematics system. A switch IC serves the simple task of switching the connection to the antenna between the transmit and receive circuits of the radio. However, finding a switch that has low distortion at high power levels, and low insertion loss, over a broad frequency range, is no simple task. Many switches increase design complexity by requiring external tuning circuits, presenting asymmetrical switch paths, or offering degraded linearity at lower supply voltages.

Fortunately, a new generation of RF switches, exemplified by the model AWS5532 switch IC from ANADIGICS, is better suited to telematics applications. It offers simplified external circuit design and improved performance. The table provides a brief overview of the advantages the new switch offers compared to a representative competing switch.

For telematics applications, linearity performance must be maintained under a variety of conditions. For example, the capability to achieve a 0.1-dB compression point above +40 dBm ensures that good linearity will be maintained at higher power levels. Compression levels should also be guaranteed at low control voltages. While regulated control voltages of 2.7 V are common in telematics applications, some RF switches only guarantee performance down to 3 V, resulting in degraded linearity when operating at voltages below that point. For example, the AWS5532 IC switch is designed to maintain consistent gain compression at required minimum control voltages (Fig. 1).

A high intercept point is also a measure of a switch well suitedfor telematics, indicating the capability to handle high signal levels with minimal generation of additional signal products or distortion. For every 1-dB increase in input signal power, the third-order intermodulation products increase by 3 dB, and the imaginary point at which these two lines intersect is the third-order intercept point (IP3). Maintaining linear ouputs at high power levels helps achieve a suitably high IP3 with good spectral purity.

Many telematics antenna switches have asymmetric paths, one for highpower transmitted signals and one for low-power received signals. These paths are not interchangeable. The receive path may be rated for power levels only as high as +19 dBm and will exhibit high insertion lossover 1 dBat higher frequencies. The AWS5532 switch has symmetric paths that can be used for either receive or transmit functions, since they both exhibit broadband responses with low insertion loss. While symmetry is not important in a single-band application, it can become an important concern in multi-band applications or those requiring antenna diversity. It is not uncommon for designers to cascade switches to achieve equivalent throws of three or more. Symmetry simplifies integration in such cases.

Existing switches often require external components to tune performance to a specific frequency band. But tuning has several drawbacks:

  • The additional capacitors and coils (inductors) consume board real estate in a space-sensitive application.
  • The additional parts (particularly the coils) are an additional expense both in procurement and in manufacturing.
  • Since each band requires a different set of components, companies must support several configurations, with engineering time, parts, and support.

The bottom line is obvious: engineers benefit from a broadband switch that does not require tuning. As shown in Fig. 2, a switch with broad frequency response requires only standard blocking capacitors on the RF ports and eliminates tuning capacitors and inductors. (Fig. 3) and (Fig. 4) show the switch's performance for two key parameters: insertion loss and port-to-port isolation.

High-power switches are also suitable for cellular-telephone handsets and similar applications. While telematics is commonly associated with in-car systems, the capabilities can also be built into mobile devices to provide "go anywhere" telematics services in "smart" phones.

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