METHODS FOR MEASURING THE POWER LINEARITY OF MICROWAVE DETECTORS FOR RADIOMETRIC APPLICATIONS

A microwave radiometer relies on the power linearity of its microwave receivers to accurately measure the temperature of remote microwave no ise sources, This paper considers linearity issues in the design and c haracterization of such receivers. Analysis is presented relating the radiometer temperature interpolation error to a second order power non linearity coefficient for the receiver, Formulas are also developed sp ecifying the temperature error in terms of individual receiver compone nt parameters, It is shown that the key parameter for the RF detector in the receiver is A(4), a fourth order RF distortion coefficient, and the key parameter for the RF amplifiers in the receiver is IP3, the t hird order intercept, This paper also discusses experimental methods f or measuring the power linearity of RF detectors to the levels require d for radiometric applications, Three methods are discussed: the two-t one method, the amplitude modulation method, and the constant ratio me thod, The theory of determining the coefficients that characterize the nonlinearity of the detector from experimental data is presented, Exp erimental results are presented showing that the two-tone method and t he constant ratio method agree to within experimental error, The sensi tivity for measuring nonlinearities and the difficulties encountered i n implementing each of these methods are also discussed.