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Controlled noise sources the key to RF test

Communications experts have long correlated symbol constellations-phase and amplitude shifts in the carrier signal-with bit error rate (BER). If BER creeps up, you can't interpret your signal. Unpredictable shifts in phase and amplitude (and frequency) are difficult enough to account for when the carrier transverses a cable or backplane; they are much more difficult to measure when, launched into the atmosphere, they have to find their way around walls and buildings.

Here, contributors offer insights into RF and related test issues that plague communications systems designers. If you want to gauge the effects of fading and noise on the BER of your cell phone or wireless-LAN hookup, you need an ultraprecise source of RF noise. As Scott Siclari of RF test-equipment maker Aeroflex Inc. notes, you're sending your RF test equipment on a wild goose chase: You can spend literally days and weeks of test time looking for noise you can't measure and bit errors that don't show up.

Injecting noise

Agilent Technologies' Noah Schmitz describes the variations on RF signal fading for both long distances and short reflections about the receiver. The best way to account for fading, he suggests, is to inject noise into your test setup.

As Aeroflex' Siclari observes, the noise you inject and its effects on BER are often an educated guess. Guess wrong and you manufacture a system with no tolerance for Gaussian noise, or, conversely, you build one that's impossible to test. BER plotted against a decreasing noise axis will generally trail off in what is called a waterfall pattern, Siclari says. The pattern is based on statistics, and the best estimates of BER will reflect a “confidence level;” that is, the best guess on how closely the measurements approximate the waterfall pattern. The formulas will be posted online with the complete text of Siclari's article on PlanetAnalog.com.

Raimondo Sessego of Amkor describes parameters an RF test station should measure, such as frequency-dependent power envelopes for IEEE 802.11 WLANs and phase shifts for 3G signal carriers.

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