Design And Implement Of K-band Microwave Radiometer With A Fast Fourier Transform Spectrum Analysis Bankend

Water vapor（H₂O） is the key constituent of the middle atmosphere. It plays a major role of the Earth’s atmosphere by emitting and absorbing in the infrared domain. Microwave radiometry measures the weak radiation emitted by rotational transitions of the molecules in the atmosphere, the spectral line of such transitions is pressure broadened, which allows one to retrieve an altitude profile of the species under investigation. Spectrometers from microwaves to sub-millimeter waves are used to detect and to measure molecular lines ubiquitous in planetary and cometary atmospheres. Today these spectrometers offer useable coherent bandwidth from a few MHz to 1-2GHz, with a few thousand spectral channels capable of resolving narrow spectral lines of masters and the thermal line emission of gaseous clouds. The radiometer with spectrum analysis backend can be considered prototypical for spectrometer development for future radio astronomical applications. The main contents include:（1）This paper presents the principle and implement of the K-band microwave radiometer with a digital Fast Fourier Transform（FFT） spectrum analysis backend. This system mainly composes a ground-based microwave heterodyne receiver and a digital spectrum analysis backend based on Fast Fourier Transform algorithm. The ground-based microwave heterodyne receiver measures H2 O transition line at 22.235 GHz and converts the intermediate-frequency（IF） signal to a complex I/Q-format, and reduces the sample rate by a factor of two, which eases the subsequent processing requirements. The design and compliment of the digital Fast Fourier Transform spectrum analysis backend is based on a commercially Kintex-7 family Evaluation Field Programmable Gate Array（FPGA） with a 4 DSP FMC150 daughter card.（2） This paper has described the architecture of the digital Fast Fourier Transform spectrum analysis backend based on FPGA and presented its software design flow about the main compositions in detail. It mainly includes the data transmission and auto-calibration module; SPI communication module; MMCM clocking manager; fixed-point to floating-point module; FFT and power spectrum calculation module, DPRAM module. All the designs have been successfully simulated with Modelsim and Chipscope.（3） Finally we have set up the K-band microwave radiometer system with a digital Fast Fourier Transform（FFT） spectrum analysis backend and measured the H2 O spectral line at 22.235 GHz.. The spectrometer works continuously and has a total bandwidth of 250 MHz, resolved into 16384 channels with a frequency resolution of 15 k Hz, with no gaps in the data. The FPGA technology has implemented the design of low power consumption and system miniaturization, which makes it easy to transplant this system. The system also has strong adaptability and extensibility, which provides the foundation and technology to enhance the ground-based microwave radiometer with a digital Fast Fourier Transform（FFT） spectrum analysis backend in microwaves to sub-millimeter waves in the future.