Radiometer receiver linearity characterization by the intermodulation distortion method

Radiometer receivers are linear systems relating the input scene temperature to the output video signal. The author provides the designer with attainable parameters to characterize the end-to-end receiver linearity, both in terms of prediction and testing. The receivers are usually calibrated by the hot/cold matched load method; hence, the linearity of the receiver directly affects the system accuracy. At the instrument level, the specification is expressed by the end-to-end receiver gain error. This figure should be in the range of 0.01% to reach a radiometric temperature accuracy of about 0.1 K. However, the receiver designer faces the problem of predicting and measuring this performance. The nonlinearity causes can be divided into RF and video. Links are established between the gain linearity and the intermodulation distortion parameters, the latter being far more accessible in the highly linear region. The intermodulation distortion approach, classically used for RF linear devices, is extended to square law detectors. The analysis assumes a Gaussian noise input signal, the receiver gain being expanded in series. Formulas for derivation of the desirable intercept points are provided.<>