Microwave Engineering Europe - April 2008 - (Page 28)

28 ZigBee How do you test ZigBee transmitters? Test suite for ZigBee includes measuring signal transmissions and ensuring compliance to IEEE 802.15.4 By David A. Hall, RF Instruments Product Marketing Manager, National Instruments Z igBee (IEEE 802.15.4) is a wireless standard for personal area network sensor monitoring and control that is designed for low-power, short-range communications between wireless devices. The standard is classiﬁed as a wireless PAN (WiPAN), a category that also includes Bluetooth (IEEE 802.15.3). The ZigBee standard has seen increasing interest from both commercial and military markets for applications such as wireless sensor networks, home automation and industrial control. One interesting facet of the ZigBee standard is that it is designed so that devices can make a self-forming and selfhealing network as needed. In this scenario, a central ‘PAN coordinator’ device oversees the health of the network conﬁguration. In recent years, sensor networks have been the subject of much research in military applications. Thus, there is signiﬁcant interest in using the ZigBee standard to deﬁne the communications links in ad hoc battleﬁeld intelligence scenarios. One design decision of the ZigBee speciﬁcation that makes it ideal for remote wireless sensors is the implementation of a low-power physical layer (PHY). The PHY speciﬁcations allow ZigBee devices to operate in one of three bands: 868 MHz (Europe), 915 MHz (North America) and 2.4 GHz (worldwide). The 2.4-GHz band, in which ZigBee transceivers are most commonly deployed, uses the offset quadrature phase-shiftkeyed (OQPSK) modulation scheme. This is a derivation of traditional QPSK and is used because it requires less power than similar schemes while achieving the same level, or better, of throughput. OQPSK uses a maximum phase transition of 90° from one symbol to the next, preventing symbol overshoot and requiring slightly less transmission power than the traditional QPSK modulation scheme. This design decision, combined with the use of a 5-MHz channel A frequency mask (white line) is compared with the output power and represents the limit of power the transmitter can emit to adjacent bands. bandwidth, enables devices to achieve a data rate of up to 250 kbits/second in a reasonably power-efﬁcient manner. Because ZigBee transceivers are designed for low-power apps, the PHY tolerates error vector magnitudes (EVMs) of up to 35 percent while maintaining reasonable bit-error-rate (BER) performance. Thus, design validation requires a variety of test methodologies. National Instruments alliance partner SeaSolve has developed a test suite that includes transmit (Tx), receive (Rx) and compliance testing for ZigBee. When testing a ZigBee transceiver’s Tx signal quality, a vector signal analyzer (VSA) must be used to characterize both spectrum information and modulated signal quality. With the SeaSolve’s WiPAN LVSA Signal Analysis tool set and a PXI-5660 VSA, both spectrum and modulation measurements can be performed on IEEE 802.15.4compliant signals. Each measurement type is a requirement for both design validation and production test. The spectral emissions of a ZigBee transmitter will dictate its interoperability with other devices in the industrial, scientiﬁc and medical (ISM) band. In addition, the modulation quality of the Tx signal, combined with the antenna performance, dictates the range of distance over which the device can reliably perform. The most common spectral measurements performed include power spectral density, occupied bandwidth, power in upper and lower bands, and total power in band. Modulation analysis tools include constellation plot, eye diagram, complementary cumulative distribution function curve and returned bit stream. Typical modulation measurements are EVM, frequency offset and BER. Various stages of product development require different measurements or analysis. The design validation and veriﬁcation stage requires more-intensive analysis tools, such as a constellation plot, to debug various issues. Production test requires more- Microwave Engineering Europe ● April 2008 ● www.mwee.com 028-029_MWEE.indd 28 27/03/08 11:01:09 http://www.mwee.com

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Microwave Engineering Europe - April 2008 News Contents Comment Test and Measurement: Comprehensive WiMAX and Wi-Fi Product Design Demands Effective Channel Emulation Military/Aerospace Focus: Hardware Needs Limit Software Radio Interview — Mitsubishi Electric Europe: GaAs Technologies Spanning High-End Space and Radar Through to Cost-Sensitive Handset and LNB Applications How Do You Test ZigBee Transmitters? Advanced Receiver Design Boosts Performance CMOS PAs Pave the Way for One-Chip Phones Products Calendar

Microwave Engineering Europe - April 2008

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