High Accuracy Signal/Noise Measurement of Microwave Power Amplifiers

To improve the repeatability and correlation of signal-to-noise ratio measurements of microwave power tubes between test sites, and between test sets at the same test site, a self-calibrating measurement technique has been developed. This technique results in an accuracy of better than 0.1 dB, and a repeatability primarily determined by the number of independent noise samples taken. Without calibration, errors of ±1.5 dB were typical in prior techniques. Accuracy was limited by vector demodulator errors (I/Q gain imbalance, quadrature phase error, and DC offsets) which had to be removed by a lengthy calibration procedure. The new procedure removes these errors by implementing an LO offset frequency technique, and digital signal processing of the video data. It also uses digital filters to create a precise noise bandwidth, and to significantly reduce the noise floor of the measurement. This new technique is being implemented in test stations at various sites, both government and industry. The improved accuracy results in far better agreement between tube vendors, equipment integrators and the final customer, leading to lower costs by increasing equipment acceptance.