Analysis of the microwave self-noise influencing on the parameters of the Dicke radiometer
Keywords:Dicke radiometer, microwave noise, measurement error, microwave switch, mismatch factor
Introduction. The application domains of the radiometric measuring are described and the basic types of radiometers are listed. The best "price – measurement precision" index (in the authors' opinion ) has a Dicke radiometer. The microwave self-noise level of this radiometer very affects on the measurement precision. It is a function of the waveguide devices characteristics, generally, of the microwave switch parameters. Microwave noise sources of the radiometer. A mixer and heterodyne are the basic sources of microwave noises in a radiometer. Analysis of the self-noise influence on the measurement precision of the radiometer with pin-modulator in the switching mode. The considered radiometer involves a pin-diodes switch. The theoretical and experimental investigations show, that the level of microwave noise depends on the channels nonidentity of the switch, and also on mismatch of its ports. Abatement of noise is possible due to reduction of the heterodyne noise factor of the radiometer, antennas noise, and also reduction of the input and output switch mismatch factors. Analysis of the self-noise influencing on the measurement precision of the radiometer with pin-modulator in the commutation mode. The results of experimental determination of the basic radiometer characteristics in this mode are described. The main devices of the radiometer waveguide transmission line are analyzed. Their influence on the input power measurement precision is shown. The computation data of the measurement error are cited for the concrete parameters of the microwave devices. In the event that a relation a "signal – noise" equals 1 and the switch coefficient of reflection is 0,35-0,55, the error can equal 18%. It is suggested to decrease considerably this error through an additional ferrite isolator and using the switches with similar waveguide channels. Conclusion. Comparison of the offered methods shows that adding ferrite isolator and using the switch with similar waveguide channels are preferential. That allows ignoring the microwave self-nose influence on the measurement precision.
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