Research On Front Ends Of Millimeter Wave Radiometers

The law of blackbody radiation enlightens people to observe matter based on this nature.Microwave radiometers are not affected by the weather,and they do not radiate energy outside,which makes them play a huge role in military detection,public place security inspection,satellite imaging and other fields.Ka and W bands are two atmospheric windows in the millimeter wave band.It has important practical value to develop two sets of radiometers in these two bands.In order to better grasp the operation mechanism of the radiometer,the physical and mathematical principles behind the microwave radiometer are introduced.After comparing some kinds of calibration methods,combining the real-time calibration method with the direct detection structure,the real-time calibration radiometer in this paper is proposed,which can fundamentally reduce the influence of gain fluctuation and local noise fluctuation on the sensitivity of radiometer.The input of radiometer adopts SP3 T with high performance,which sacrifices a little system noise to ensure the real-time calibration function.The main design process includes device selection,scheme demonstration and circuit design.For Ka-band radiometer,according to the previous research and development experience of millimeter wave detector,the gain requirements of low-noise amplifier are deduced from the linear detection range of millimeter wave dector,and the noise coefficient requirements of low-noise amplifier are determined by the sensitivity requirements of radiometer.The appropriate low-noise amplifier chip and SP3 T switch chip are comprehensively selected to accurately estimate the architecture to meet the requirements.In the circuit design stage,the power supply design of low noise amplifier,simulation and optimization of detector,transition structure design,cavity design,video amplifier circuit and PCB design of power supply part are respectively designed,and the circuit layout and cavity layout are carried out from the perspective of electromagnetic compatibility.The design process of W-band radiometer is similar to this,but because of the poor out of band suppression of its low-noise amplifier chip,the ultra wideband bandpass filter of 80~112GHz is designed.After investigating various classical design methods,such as fin line filter and cavity filter,the disadvantages and structural constraints of the traditional design methods are analyzed,and the structural design method is derived,combining the simple waveguide high pass structure and the structure of band stop filter with good rejection.The design efficiency can be greatly improved.The structure is simple and the performance is good.Two sets of radiometers,Ka band and W band,were fabricated and tested.The test results show that when the integration time of Ka band radiometer is 0.27 ms,the brightness temperature sensitivity of Ka band radiometer is 0.34 K.When the integration time of W-band radiometer is 0.27 ms,the sensitivity is 0.39 K.The millimeter wave radiometers designed in this paper have excellent performance and meet the requirements of the project.