Technologies Of Chirp Transform Spectrometer For Deep Space Exploration

Deep space exploration,which studies the origin and evolution of the universe by detecting the spectrum lines of interstellar material on the surface of planets and small celestial bodies and in the nearby atmosphere,has attracted the attention of the space science communities.Among the many objects detected in deep space,detecting water and carbon monoxide is important for the understanding of the composition of asteroids,which contribute to find the basic conditions for life to exist.The detection of the isotopic ratios of water,at the same time,is also important in determining the source of diagenetic minerals.The detection of ammonia gas is very important for the study of nitrogen excited states in the solar nebula.Many of these interstellar material absorption spectral lines are in the terahertz band,therefore it is crucial to use terahertz spectrometer in deep space exploration.The importance of hyperfine spectral line observation is highlighted by the very small frequency intervals of some important probe lines.It is necessary to develop high bandwidth and high resolution terahertz spectrometer for fine spectrum detection.In the realization of the back-end spectrum analysis technology of spaceborne terahertz spectrometer,the Chirp transform spectrometer based on surface acoustic wave filter is suitable for deep space exploration due to its characteristics of radiation resistance,high stability and low power consumption.This thesis is focused on the key technology for spectrum analyzer back-end system According to the principle of Chirp transformation,the input signal's energy spectrum can be obtained by multiplying it with the Chirp signal generated by expander and then convolving with he Chirp signal generated by the compressor.Then,two operation forms M(s)-C(l)and M(l)-C(s)are analyzed,compared and selected respectively,and the overall structure of the spectrum analyzer is designed based on the selected form.There are two methods to achieve pulse compression: analog pulse compression and digital pulse compression.By comparing the two methods,it can be seen that the combination of the two methods,that is,Chirp signal with large bandwidth generated by digital technology and pulse compression realized by acoustic surface wave filter,can give full play to the advantages of the two methods and optimize the system performance.Then,according to the development of surface acoustic wave filter in China,the main indexes and system design scheme of the Chirp transform spectrum analyzer,which is used for spectral detection of water and its isotopes,are presented in this paper.The compressor is realized by stimulating acoustic surface wave filter with 400 MHz bandwidth and 10?s dispersion time,which is developed by the Institute of Acoustics of Chinese academy of sciences.The spread spectrum unit generates the required high-bandwidth Chirp signal with the AD9914 direct digital syntheor chip controlled by FPGA.Then,the expander was built,and the signal observed in the spectrometer meets the requirement.Under system integration,the signals of different frequencies are input at the signal terminal to be tested.The phenomenon of pulse compression signal changing with the input signal frequency changes is observed on the oscilloscope,which symbolizes the basic function of the spectrum analyzer is realized.However,the frequency resolution is failed to meet the design target.After analysis,the output signal of the mixer is mixed with the input signal.In order to obtain a relatively pure Chirp signal,the system is improved by applying high-pass filtering after up-conversing.The frequency resolution of the improved spectrometer is 152 k Hz,which satisfies the system index,but is still insufficient to the theoretical value.Then matching conditions of pulse compression are analyzed,and a Chirp transform model is established in MATLAB software.After the ideal model and the actual model are compared,amplitude compensation curve,phase compensation curve and pulse compression curve with compensating are given.After amplitude compensation and phase compensation,the resolution of the system reaches 108 k Hz,which is very close to the theoretical value.