Analysis And Experimental Validation Of Multi-channel Bandwidth Synthetic Radiometer

The microwave radiometer is a highly sensitive receiver capable of measuring very low levels of microwave radiation. The sensitivity is one of the most important parameters of a radiometer. For a traditional single channel radiometer, the sensitivity can be improved to some extent by using high performance devices. It also enlarges the predetection bandwidth to meet certain applications. Therefore, it is of paramount importance to improve the sensitivity by modifying the system circuit structure for the development and application of radiometers. Aiming to improve the radiometer sensitivity, this dissertation proposes the bandwidth synthetic method. Moreover, in order to realize the miniaturization and integration of the whole radiometer system, researches are conducted on the multi-band bandpass filters and waveguide-microstrip transition with filtering characteristic. The following aspects are discussed in the dissertation:According to the analysis of the benchmarked traditional radiometers’ characteristics and the limitation on the improvement of radiometer sensitivity, the bandwidth synthetic method is proposed in this dissertation. This method improves the radiometer sensitivity by utilizing a wider bandwidth which synthesized by using multiple frequency bands. In this dissertation, the principle and realization of the multi-band bandwidth synthesis radiometer is described in detail. The frequencies transformation is also explained by an example of the dual-band bandwidth synthesis radiometer. The multi-channel bandwidth synthesis radiometer(MCBWSR) sensitivity formula is derived. In order to analyze the relationship between MCBWSR sensitivity and the traditional single channel radiometers’, the formula of MCBWSR sensitivity expressed by traditional radiometer sensitivity is further deduced. For validation of the multi-band bandwidth synthesis radiometer sensitivity, the signal processing theory is adopted.Based on the theoretical analysis on the MCBWSR, the simulation and experiment on MCBWSR are implemented in this dissertation. For the simulation, the MCBWSR system model and related device models are built at first. To be compared with MCBWSR, the system models are also built to describe the traditional single channel direct detection radiometer and superheterodyne radiometer. Secondly, two dual-channel bandwidth synthesis radiometer models(one consists of two single channel direct detection radiometers while the other is composed of a single channel direct detection radiometer and a single channel supterheterodyne radiometer) and a tri-channel bandwidth synthesis radiometer system simulation model are built. The simulated system sensitivities are obtained and compared with the theoretical calculated ones. At last, the comparison of system sensitivity between MCBWSR and traditional single channel radiometer is given, validating the correction of the MCBWSR sensitivity formula. To experimentally verify the theory, the related devices are designed, fabricated and measured according to the requirement of both single channel radiometer and MCBWSR system individually. Next a single channel direct detection radiometer, a single channel superheterodyne radiometer and a dual-band bandwidth synthesis radiometer experimental platform are built individually. The system sensitivities of the experimental system are obtained with calibration. Finally, the MCBWSR sensitivity formula is further validated by comparing the sensitivity of single channel direct detection radiometer, single channel superheterodyme radiometer and dual-band bandwidth synthesis radiometer.Because of the limitation of the reported radiometer model, none of them is accurate when simulating the real wideband radiometer system, for example the system gain and equivalent noise factor fluctuations in frequency domain. A new model for wideband radiometer system is proposed in this dissertation. This model accurately captures the real conditions(i.e. the system gain fluctuation, noise factor fluctuation and the change of the operation temperature) and the calibration of wideband radiometer systems. Frequency domain parameters(such as the equivalent bandwidth) can be obtained with the frequency domain simulation, while and time domain parameters(for example calibration for the radiometer sensitivity) can be acquired with time domain simulation. To verify the influence of the noise factor fluctuation in frequency domain, an equivalent noise factor model is proposed according to the theory of cascaded noise factor and amplifier simulating the noise factor fluctuation in the whole operating frequency. Using the equivalent noise factor model and wideband radiometer system model, the relationship between the radiometer sensitivity and the noise factor fluctuation in frequency domain is simulated. The influence of the system gain fluctuation on the equivalent bandwidth and the radiometer sensitivity cannot be ignored. The relationship between the equivalent bandwidth and radiometer sensitivity is validated with the wideband radiometer simulation model, equivalent noise factor model and the equivalent bandwidth model. Finally, to verify the theory, an experiment is implemented. Meanwhile, the experiment results are also given and compared with the calculated ones.In this dissertation, several integrated and miniaturized planar multi-band bandpass filters(BPFs), waveguide fin-line BPF and waveguide-microstrip transition with filtering characteristics are proposed. For planar filters, firstly, the stub-loaded and stepped impedance resonators are analyzed. According to the theory analysis, several miniaturized wideband BPF, dual-band BPF, tri-band BPF, Quad-band BPF, ultra-wideband BPF and ultra-wideband with notch band are proposed based on the different resonators. Secondly, the method using a same resonator to design wideband BPF, dual-band BPF and tri-band BPF is proposed. Meanwhile, the theory analysis and design examples are given. For waveguide filters, three W band waveguide fin-line wideband BPF with different operation frequency are proposed. This type of filters is easy to realize and to integrate with other system. For further improving the radiometer system integration and miniaturization performance, a waveguide-to-microstrip transition with filtering characteristics is proposed based on the waveguide BPF and waveguide-to-microstrip transition. This device is designed for filtering and transition under the condition not to increase the device volume which realizes the conception one device has multiple functions. Meanwhile, multiple transmission zeroes are introduced in the stopband improving the selectivity and rejection of the stopband.