ZigBee (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 self-healing 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-shift-keyed (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 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) per- formance. Thus, design validation requires a variety of test methodologies.
A frequency mask (white line) is compared with the output power and represents the limit of power the transmitter can emit to adjacent bands. |
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 802.15.4-compliant 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-definitive measurements, such as EVM and frequency offset.
ZigBee Tx spectrum analysis
Power spectral density describes how the power of a given packet is spread over a broad frequency range. Measuring PSD ensures that the transmitter operates within the spectral mask requirements of the IEEE 802.15.4 standard. A frequency mask is compared with the output power. The frequency mask represents the power the transmitter is allowed to emit into adjacent bands. When troubleshooting a device, factors such as poor filter design or images resulting from amplifier compression can contribute to un- wanted power in adjacent bands.
While EVM enables the capture of various impairments with a numeric value, the constellation plot enables a visual ID of the error source, as seen here, where the in-phase and quadrature-phase components of the local oscillator are not precisely 90° out of phase. |
The power-in-band measurement calculates the integrated power (dBm) in the specified channel or band. Measuring power in band ensures that the transmitter does not exceed IEEE 802.15.4 specs.
Occupied bandwidth measures the bandwidth of the specified frequency band that contains 99 percent percent of the total power of the span.
Adjacent channel power measurement comprises power in the upper and lower bands. According to IEEE 802.15.4, the upper band is 5 MHz toward the right of the operating frequency, and the lower band is 5 MHz toward the left.
Baseband parametric measurements en- sure that the receiver can successfully decode ZigBee transmit packets. Because ZigBee transceivers are designed to operate at low power and do not require high data throughput, modulation quality is often sacrificed to reduce power consumption. Overall, the purpose of measuring quality is to evaluate the likelihood of bit errors. As an example, BER can be estimated as a function of EVM (in terms of percentage). BER increases dramatically when the EVM of a QPSK transceiver increases from 15 percent to 30 percent. By contrast, most ZigBee devices are required to operate at an EVM below 35 percent. To ensure that a transceiver will operate effectively in its deployment environment, it is important to measure modulation accuracy, which can be done using several plots and measurements.
Measuring EVM enables designers to capture various problems and impairments, such as local oscillator (LO) stability, quality of the intermediate frequency (IF) filter, compression, symbol rate and interfering tones. By measuring EVM, linearity and efficiency can be verified. During analysis, the user can check whether EVM always falls below the standard specified reference of 35 percent, which ensures good demodulation of the transmitted signals.
Typically, EVM is measured both on a per-symbol basis and as an RMS EVM percentage, which captures the average EVM for the entire packet.
The constellation plot provides a graphic representation of the demodulated baseband waveform. This diagram is one of the most valuable during the design validation stage because it can be used to identify problems such as IQ gain imbalance, dc offset and quadrature skew. Unlike the EVM measurement, which provides a simple numeric value, the constellation plot also provides a visual representation of the source of error.
Although EVM provides a specific mechanism of quantifying impairments, the size and shape of the constellation plot provide a visible indication of the type of impairment that is present.
The eye diagram also reveals the modulation characteristics of a Tx signal. In contrast with the constellation plot, it provides a time-domain view of the signal and can be used to visualize shaping or channel distortions.
Using this measurement, designers can determine the optimum sampling point and decision for decoding the data. During analysis, users can check for the maximum eye openings in the signal after offset removal (OQPSK > QPSK) to validate demodulation properties.
One of the most common metrics for quantifying receiver performance is to measure BER. Because low EVM results in errors that occur infrequently, this measurement can be time-consuming, depending on the modulation quality. As a result, extended BER tests are most commonly performed during the design validation phase.
In a production test, a much shorter BER test is used. BER measurements can be made by returning the decoded raw data as a stream of ones and zeros. When these values are compared with a known transmission, BER can be calculated.
Complementary cumulative distribution function (CCDF) is used to analyze the power characteristics of a signal. As discussed earlier, the ZigBee specification requires the use of the OQPSK modulation scheme to minimize power requirements. The power efficiency of the transmitter is maximized when the Tx power is constant. The CCDF curve can be used to verify that power fluctuations do not occur and can be used to represent the percentage of power above the average power. Ideally, the right edge of the CCDF curve is perfectly vertical. In this scenario, a power amp can maintain the highest power efficiency without being driven into saturation.
David A. Hall (firstname.lastname@example.org) is RF instruments product marketing manager at National Instruments. He has expertise in DSP, digital communications and RF measurements. Hall holds a BS from Penn State University.