RF Antenna Switch Simplifies Radios in Telematics

JasonYoo

RF Antenna Switch Simplifies Radios in Telematics [Copy link]

Telematics is the combination of computer technology and communication technology, which is being applied to several fast-growing markets, including the automotive industry. Telematics supports remote diagnostics, on-demand navigation, emergency assistance, speeding or stolen vehicle tracking, and intelligent transportation systems.

As automotive telematics devices become more common, designers face the same challenges as with any communications evolution: increasing functionality while reducing cost. Although telematics can draw on experience with wireless phones and wireless local area networks ( WLANs ), the technology has special requirements that cannot be easily met by advanced integrated circuits (ICs) designed for wireless handsets and WLAN adapters. Engineers designing high-performance, cost-effective telematics radios prefer devices that have been optimized for the specific requirements of telematics applications.

For example, let's look at the function of an RF switch in a telematics system . The switch IC performs the simple task of switching the connection to the antenna between the radio transmit and receive circuits . However, finding a switch with low losses at high power levels and low insertion loss over a wide frequency range is not a simple task. Many switches require additional conditioning circuitry, exhibit asymmetrical switching paths, or have reduced linearity at lower supply voltages, adding complexity.

Fortunately, a new generation of RF switches, such as the AWS5532 switch IC from ANADIGICS, is more suitable for telematics applications. It simplifies external circuit design and improves performance. The table briefly summarizes the advantages of the new switch compared to representative competitive switch products.

For telematics applications, linear performance must be maintained under a wide range of conditions. For example, the ability to achieve a 0.1-dB compression point of more than +40 dBm ensures good linearity at higher power levels. Compression levels must also be guaranteed at low control voltages. Although a stable control voltage of 2.7 V is common in telematics applications, some RF switches only guarantee performance down to 3 V, with linearity performance degrading when operating below that point. For example, the AWS5532 IC switch is designed to maintain constant gain compression at the required minimum control voltage (Figure 1).

A high cutoff point is also a measure of whether a switch is well suited for telematics and is a sign of its ability to handle high signal levels with minimal additional signal or distortion. For every 1-dB drop in input signal power, third-order intermodulation increases by 3dB, and the point where the two imaginary lines cross is the third-order cutoff point (IP3). Maintaining a linear output at high power levels helps achieve a suitably high IP3 with good spectral purity.

Many telematics antenna switches have asymmetric paths, one for high power transmit signals and the other for low power receive signals. These paths are not interchangeable. The receive path only needs to reach power levels of +19dBm and exhibits high insertion loss of more than 1dB at higher frequencies. The AWS5532 switch has symmetrical paths and can be used for both receive and transmit functions because they both exhibit wideband response with low insertion loss. While symmetry is not critical in single-band applications, it is an important consideration in multi-band applications or applications that require antenna diversity. Designers do not often cascade switches to achieve 3 or more equivalent solutions. In this case, symmetry simplifies integration.

Existing switches often require external components to adjust performance for a specific frequency band. But tuning has several disadvantages: In space-sensitive applications, additional capacitors and coils (inductors) consume board real estate; additional parts (especially coils) are an additional expense in procurement and manufacturing; and because each frequency band requires different components, companies must support several configurations, including engineering time, parts, and support.

The bottom line is clear: engineers benefit from broadband switches that do not require tuning. As shown in Figure 2, switches with broadband frequency response require only standard DC blocking capacitors at the RF port and do not require tuning capacitors and inductors. Figures 3 and 4 show two key parameters of switch performance: insertion loss and port-to-port isolation.

High power switches are also suitable for cellular handsets and similar applications. While telematics is usually associated with in-vehicle systems, these capabilities can also be integrated into mobile devices to provide “anywhere” telematics services in “smart” phones.