Microwave noise measurement errors caused by frequency discrepancies and nonzero bandwidth

We develop analytical expressions for two frequency-related errors encountered in noise parameter measurements of devices with high-reflection coefficients such as GaAs FET's and HEMT's. The first error is caused by a discrepancy between the measurement frequency of the noise-measuring receiver and the frequency of the reflection coefficient-measuring apparatus. The second is caused by variation of the noise power spectral density across the bandwidth of the noise-measuring receiver. A noise power measurement on a cooled GaAs FET is presented as a demonstration, in which the noise power level exhibits narrow peaks which rise about 10 dB above the general level at regular frequency intervals. It is shown that the severity of both of the frequency-related errors is related to the shape of these peaks. Simple expressions are derived which allow the severity of these two types of errors to be estimated when values are available for the reflection coefficient magnitudes and transmission line lengths in a system. Expressions are also derived which give the variation with frequency of the measured noise power and also of the two error terms, if the noise parameters of the device under test are also available. This analysis accounts successfully for the main features of the experimental noise power observation.<
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