Study Of Spectrum Detection With High SNR And Adaptive Gain Based On OSK Modulation

In view of the narrow spectrum, small input dynamic range and lack of signal-to-noise ratio(SNR) in spectrum detection, the adaptive gain spectrum detection technology with high SNR, wide spectrum and large input dynamic range is explored in this thesis, which is based on the requirements of high performance spectrum detection by summarizing the technology progress and application status of spectrum analysis instruments. In order to improve the result of spectrum detection from the source side of the system, this dissertation mainly studies the spectrum detection technique with high SNR and adaptive-gain based on OSK modulating according to the electronics design. On this basis, the characteristics of spectrum component, design of adaptive gain and methods of improving SNR are the key research contents, and finally the design and realization of the spectrum detecting principle prototype is completed.The high SNR and adaptive gain spectrum detection system based on OSK modulating adopts acousto-optic tunable filter(AOTF) as the electronically tunable beam splitting device, uses the direct digital synthesizer(DDS) AD9910 to generate a radio frequency(RF) signal to achieve a series of rapid, continuous and stable spectrum. To achieve the function of automatic gain control in the spectrum detection system, methods of judgment of sampling points and closed loop gain shaping control(CLGC) are adopted in the circuit based on the combination of programmed gain amplifier(PGA) AD8253 and voltage-controlled amplifier(VCA) AD603. Based on the electronically tunable character of AOTF crystal, switching RF signal on and off by using the output shift keying(OSK) function of AD9910 to achieve the AC spectral signal. This stated signal is processed by the lock-in amplifier technique along with narrow-band filtering, achieving high output SNR in spectrum detection.The results of testing the spectrum detecting principle prototype which has advantages of high SNR and adaptive gain show that: ①The adaptive gain of the system ranges from 9.48 to 7536, and the manually gain adjustment range can be up to ?9.48 ~ ?119430; ②The output SNR of the system can be more than 55 d B, when testing a small signal without preamplifier, and the SNR of the principle prototype can be more than 40 d B, when testing a black body with preamplifier. And the inversion error of the system based on the input and output data is less than 8%. Compared with the Chang’E 3’s(CE-3’s) payload infrared spectrometer, this prototype owns much bigger input dynamic range, wider spectrum detection range and higher output SNR, and its inversion error is also reduced.Overall, the system has accomplished the research target and contents well, and that the stated research result has been adopted in application of “CE-5’s Payload Lunar Mineral Spectrum Analyzer” and “AOTF Performance Detecting System With Wide Spectrum”.