Microwave Engineering Europe - April 2008 - (Page 29)

ZigBee 29 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 unwanted power in adjacent bands. 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 ensure 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 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 quadraturephase components of the local oscillator are not precisely 90° out of phase. 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. Eye diagram The eye diagram also reveals the modulation characteristics of a Tx signal. In contrast with the constellation plot, it provides a timedomain 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 (david.hall@ni.com) is RF instruments product marketing manager at National Instruments. He has expertise in DSP, digital communications and RF measurements. Microwave Engineering ● April 2008 ● www.mwee.com 028-029_MWEE.indd 29 27/03/08 11:01:28 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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