EVM Measurements On ZigBee Signals

The most important band for the recently adopted ZigBee standard is the global 2.4 GHz ISM band, for which it has been specified with 250 kbit/s on 16 channels. But also the European band at 868 MHz and the North American ISM band at 915 MHz are used. OQPSK modulation is performed in the 2.4 GHz band, and BPSK modulation in the 868 MHz and 915 MHz bands.

Standard vector signal analyzers can demodulate BPSK-modulated ZigBee signals that have been processed with RRC filters and determine the modu-lation quality. However, this differs from OQPSK-modulated ZigBee signals because ZigBee uses a special filter with an impulse response in the form of a half sine. In this case, conventional analyzers cannot be used, since they are usually not fitted with this filter and measurement demodulation therefore does not yield useful results.

Such measurements do not pose any problems for the universal Signal Analyzer R&S FSQ from Rohde & Schwarz together with the option R&S FSQ-K70: In combination, they can load user-definable modulation filters and are thus ideal for measurements on ZigBee OQPSK-modulated signals.

OQPSK modulation is related to QPSK and operates with two bits per modulation symbol. With QPSK modulation, signal transitions occur at the origin of the I/Q diagram (zero crossings). This amplitude modulation calls for virtually linear amplifiers with correspondingly high power consumption.

RRC filters are usually used with OQPSK, and the I and Q transitions are shifted by half a symbol time to avoid such zero crossings. The special ZigBee filter eliminates amplitude modulation and converts the vector diagram of the OQPSK modulation to a circle. This makes the OQPSK modulation a modulation with constant envelopes, similar to MSK.