Used in more and more applications, from cars to home entertainment, short-range devices (SRDs) are having a growing impact on everyday life. Surface acoustic wave filters ensure high safety and security.

Remote keyless entry for cars, garage door controls, and remote controls for audio and video equipment are popular and familiar applications of SRDs. But this technology is used not only for convenience applications. For example, radio-based modules that constantly monitor pressure inside car tires and promptly warn drivers of deflation and increased temperature prevent serious accidents. SRDs are also suitable for

• Wireless headphones
• Wireless alarm sensors (glass breakage and motion detectors)
• Toll collection

Standards for frequency ranges

SAW filters provide the most effective protection of front-ends of radio receivers against overdriving by strong adjacent signals. Wanted signals in the filter passband reach the receiver virtually intact, while other frequencies and interference are strongly attenuated. A block diagram of a superhet receiver for an SRD is shown in 1.

All these short-range devices operate in the SRD or ISM (industrial, scientific and medical) bands. These license-free frequency bands are accessible at national level to the general public but have not, unfortunately, been standardized worldwide. For various local markets, manufacturers who wish to use SRDs to control their products must offer versions matching regional frequencies. This increases product complexity and costs. In Europe, the frequency band from 868 to 870 MHz ( 2) has been additionally assigned to SRDs to solve the incompatibility problem posed by different local frequencies. In the United States, this frequency band is already used for mobile telephony (CDMA), and the range from 902 L to 928 MHz has been reserved for SRDs.

In the new frequency bands to be used in the future, there is a trend toward higher frequencies in the gigahertz range because the spectrum below 1 GHz is largely exhausted. Depending on application and technology, there will be a tendency to use the higher frequency bands or the traditional ISM bands with center frequencies of 315 or 433.92 MHz. Every band has its own regulations on output power, duty cycle, target applications and maximum radiated interference. One US company, for example, uses the 915 MHz band to locate stock in warehouses. For this purpose, a total of four receivers mounted on the four warehouse walls pick up signals emitted by small SRD transmitters in the boxes at regular intervals of several minutes. They also measure signal strength. By comparing the signal strength at all four receivers, the system can determine the location of any box in the warehouse with an accuracy of one meter.

Bidirectional data transmission

The SRDs described so far are prone to a security flaw inherent in the system: they are all unidirectional systems without any feedback option. The user can never be sure if data has been transmitted completely and free of errors. In many systems such as keyless entry or audio/video systems, feedback is indirect from the selected function performed.

The situation is different in applications in which the function triggered by radio remote control is not directly visible, e.g. in wireless alarm systems or telemetry. The radio receiver can only detect a failure in wireless data transmission by means of timeout algorithms. Error correction mechanisms in the bitstream can reduce data transmission errors, but residual data errors are enough to cause a system error, because the data packet cannot be requested a second time. Bidirectional systems can remedy this situation, but have several disadvantages of their own 3.

The ZigBee Alliance, an association of major hardware manufacturers, addresses this basic flaw of unidirectional data transmission. ZigBee is an emerging as an open bidirectional radio standard for energy-saving wireless data transmission over short distances and will close a technological gap. Also known as Bluetooth light, ZigBee was adopted in August 2003 for short-range monitoring and control in line with IEEE 802.15.4 and focuses on very low power consumption in battery-powered devices. What's more, as its system structure is simpler than that of Bluetooth solutions, semiconductor costs are reduced. Typical applications will be found in household appliances and medical engineering.

The standard is based on a simple but powerful protocol with high transfer reliability. ZigBee is distinguished by

• Feedback
• Error checking
• Arbitrary frequency hopping to take advantage of low-interference channels
• Definable security stages
• Worldwide availability thanks to license-free frequency bands of 868 MHz (Europe), 915 MHz (USA) and 2.4 GHz (worldwide)

The 2.4 GHz frequency band is completely divided into 16 channels with a channel spacing of 5 MHz. The 868.3 MHz band accommodates one channel, while the 902 - 928 MHz band has ten channels with a spacing of 2 MHz 4.

IEEE 802.15.4 defines not only the physical layer (hardware), but also the media access control layers above it. On this basis, the ZigBee Alliance is defining the wireless network platform on which these devices can reliably communicate with one other. SAW filters available from EPCOS for SRDs are listed in 5.