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Power amplifier distortion test suite

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article image Screenshot from Agilent’s distortion test suite.

AGILENT Technologies has introduced a software application for its spectrum analyser families, which allows multichannel power amplifier (MCPA) designers to achieve the lowest possible distortion and highest linearity with the least amount of effort and complexity.

The Agilent 89604A distortion test suite can be used with Agilent ESA-E or PSA Series spectrum analysers or 89600 Series vector signal analysers.

When used with a spectrum analyser, the ESG or PSG Series digital signal generator, and a PC running any current Windows operating system, the 89604A delivers the processed (delay-corrected and amplitude normalised) signal files designers need to program advanced curve fitters that control the linearity of MCPAs.

The test suite also graphically shows the incremental signal degradation contributed by the MCPA, as well as many other characteristics useful for optimising amplifier performance.

The test suite and its accompanying instruments use stimulus signals that are very similar to those the amplifier will experience in service, which is one of the most important factors for the solution's measurement accuracy.

Traditional techniques use a single swept CW signal or multiple CW signals as a stimulus, which do not accurately imitate the complexity of today's broadband digital modulation schemes. As a result, amplifiers optimised using these stimulus signals can perform poorly.

In contrast, the distortion test suite works with real-world signals provided by a base station, test signals produced by a signal generator, or even Additive White Gaussian Noise (AWGN).

Thus, MCPAs characterised with the distortion test suite are much more likely to meet or exceed tight telecommunications industry standards.

This PC-based software also provides complete, time-aligned stimulus/response data, resampled to match user needs, for input to the predistortion curve fitter, which the designer would otherwise need to compile manually.

The 89604A distortion test suite uses complex modulated "real-world" signals to accurately measure AM/AM and AM/PM distortion with up to 36MHz of measurement bandwidth (depending on the instruments used), ensuring that even subtle distortion mechanisms such as memory effects and component heating are stimulated and measured.

The results are displayed together with an ideal performance line for reference and a curve fit to the results. This helps evaluate how far from ideal the MCPA moves as it approaches saturation, which occurs when the amplifier reaches beyond the highest input level it can withstand and still produce a clean signal.

The distortion test suite provides other useful information as well. The gain compression display scales the AM/AM gain curve to emphasise variations in gain, and the probability density function (PDF) curve lets the designer examine the stimulus and response signals to ensure that they have the expected distribution around their means.

The complementary cumulative distribution function (CCDF) curves show how much headroom is available in the design. The distortion test suite can also show how error vector magnitude (EVM) contributed by the MCPA degrades the modulation of the signal.

Finally, the distortion test suite provides a results display of the polynomial coefficients used by the AM/AM and AM/PM curve-fitting algorithm, which can then be used to set up the predistortion curve fitter.

Measurement setup is simple: Once a few connections are made, the user enters centre frequency, measurement span, number of measurement channels, input range, and values of external attenuators, then presses start. The distortion test suite and instruments do the rest.

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