| Radiometer is a receiver that can detect small signal power.It can distinguish objects by using the characteristics of different electromagnetic radiation emitted by different analyte.The development of radiometers have been gradually carried out since the 1960 s.As people make use of microwave frequency bands tends to higher and higher frequency,terahertz radiometers have appeared successively.This thesis focuses on the development of the direct radiometer,which is used as the intermediate frequency link of the terahertz radiometer to detect the gas content in the atmosphere.The research of multi-channel radiometer is of great significance.The multi-channels microwave radiometers are applied to receive front-end of the terahertz band,and the terahertz spectrum of matter has the property of rich physical and chemical information to identify the composition of some kinds of gas,which is of great significance for the detection of gas content,such as oxygen and water molecules.The detection of gas content such as water molecules is of great significance.The feature of the multi-channel radiometer in this thesis is that it uses different channels to detect different gases with different frequencies,so as to reduce the interference of other gases.Radiometers can be divided into Dick-type radiometers and full-power radiometers.In order to reduce the influence of gain fluctuation on the test results of brightness temperature sensitivity,a Dick-type structure is planned.According to the unique advantages of microwave,a multi-channel direct detection radiometers is developed in this project,and it is used as the intermediate frequency link of the terahertz radiometers.The direct-detection radiometer consists of a high-gain amplifier,a detector,and a video integral amplifier circuit,etc.Its multi-channel is divided into multiple narrow-band channels by a pentplexer,and then realized by detectors with different operating frequencies.Considering the whole structure of the system,the detector and the high-gain low-noise amplifier are designed into different cavities.After processing and testing,the gains of four high-gain amplifiers are about 25 d B in the range of4～15GHz,and the noise figure is less than 2.5d B;the detector bandwidth of different channels are about 1GHz.In their respective operating frequency bands,the detection sensitivities of seven channels detectors are all higher than 13.2m V/μW The video integration amplifier circuit amplifies the detector output voltage by about 1200 times,and provides the system with an integration time of 2.7ms.When the integration time is2.7ms,the brightness temperature sensitivity of each channel of the multi-channel direct detection radiometer is about 0.5K.At the same time,an F-band band-pass filter and a single-pole double-throw Dick switch are also designed.The F-band band-pass filter is used before the mixer of the 118 GHz radiometer to suppress the image frequency.The filter’s insertion loss at 114.3-122 GHz is less than 0.5d B,and the return loss is greater than 17 d B;the Dick-type switch is located behind the mixer,and the switching cycle is controlled by the data acquisition card to ensure the consistency of the system.After testing,within 4～15GHz,the insertion loss of the switch is less than 2.5d B,and the isolation is greater than 35 d B.Finally,several direct detection radiometers mentioned above are applied to the terahertz multi-channel radiometer system and tested.The two sets of terahertz radiometers operate at 118 GHz and 190 GHz.Among which,the IF of the 118 GHz radiometer is divided into 5 detection channels for output,and the IF of the 190 GHz radiometer is divided into 3 detection channels for output.The 118 GHz full-power radiometer with an integration time of 2.7ms is tested,and the brightness temperature sensitivity of each channel is about 1.2K.Due to the improvement of mixer performance,after measeuring the difficult degree between increasing the integration time of the system and changing the full-power radiometer to the Dick type,the integration time of the system is increased to about 146 ms,and the temperature sensitivity of each channel of the radiometer is about 0.5K,which meets the project requirements,so the Dick-type structure is not used in the end. |
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