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Millimeter-wave radiometer for the measurement of temperatures in hot transient plasma Carter, Charles Ruskin

Abstract

A superheterodyne Dicke type radiometer suitable for the measurement of radiation from a high temperature plasma in the 35 GHz range has been developed. The radiometer employs a balanced mixer at the radiometer frequency, a 3.5 GHz parametric amplifier using a varactor diode as the first IF amplifier, a broad-band transistor amplifier at the second IF of 7Q MHz and a commutator detector. The performance of the radiometer has been measured by conducting hot load tests and by using an S-band argon noise source. The minimum detectable temperature change was found from the hot load tests to be 11 deg K for an output bandwidth of 0.32 Hz. However, from argon noise source measurements, a minimum detectable temperature change of 1.4 x 10³ deg K was determined for an output bandwidth of 6.4 KHz and 9.5 x 10³ deg K for an output bandwidth of 160 KHz. The equation for the minimum detectable temperature change for the Dicke radiometer has been deduced following conventional analysis. It has been found that there are two errors in Goldstein's derivation the effects of which cancel out and his final formula is correct. Thus, the change suggested by Ring does not appear to be valid. For the two channel subtraction radiometer it has been found that the expression given by Graham should be multiplied by √2. The d.c. radiometer has also been analysed and it has been found that its minimum detectable signal power is independent of both the radiometer bandwidth and the output bandwidth. The effect of noise at the radiometer input before the modulating switch has been investigated and it is shown that this noise could produce a cutoff condition in the Dicke radiometer and the two channel subtraction radiometer.

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