Stability analysis of a 243 GHz direct detection radiometer

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Examensarbete för masterexamen
Master Thesis
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2018
Författare
Kjörk, Fredrik
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With a low atmospheric attenuation, a frequency band centred around 243 GHz is of interest as a reference when conducting measurements on different absorbing spectral lines of the atmosphere. A direct detection radiometer for this band had previously been constructed at Omnisys Instruments. Different aspects of the radiometer’s performance had been tested, but no satisfactory measurements of its stability had been made. This is an important measurement to conduct, since poor stability can reveal accuracy problems. The purpose of this report was therefor to evaluate the stability of that radiometer. This evaluation was done by analysing the Allan variance of the system, and finding the receiver temperature with a Y-factor measurement. The intended use of the radiometer was also replicated, by measuring an unknown temperature, along with calibrations against a hot and a cold load. The system was also compared to a commercial direct detector from VDI to compare stability, and to a down converting radiometer to determine the origin of the potential inaccuracy. The Y-factor measurement gave a receiver temperature of Trec = 1370K, and the Allan variance revealed a possible stable integration time of 10 ms before flicker noise dominates. Using that integration time, the replication test showed an inaccuracy of the radiometer of several degrees. With the use of a mixer, the flicker noise was concluded to origin in the detector diode. Since it is not practical to use switching times of 10 ms due to wear on the mechanics, it was unfortunately also concluded that the stability was not good enough. A stable integration time of at least 10 to 100 times longer would have been necessary. The comparison of the Omnisys detector with the VDI detector concluded that they were of comparable stability.
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Elektronik, Innovation och entreprenörskap (nyttiggörande), Informations- och kommunikationsteknik, Electronics, Innovation & Entrepreneurship, Information & Communication Technology
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