傅里叶变换微波光谱仪的信号采集与数据处理研究

Investigation on Signal Acquisition and Data Processing of A Fourier Transform Microwave Spectrometer

傅里叶变换微波光谱仪能精确测量分子的转动跃迁谱线,是量子化学和天文化学领域的重要实验工具。傅里叶变换微波光谱仪常利用混频器对高频微波分子信号进行下变频至低频后进行采样,并进行多次累加平均以提高信噪比。为了寻求高效、灵敏和准确的分子转动光谱检测方案,设计了零差式(Homodyne)和外差式(Heterodyne)两种不同的检测方案,提出了时域数据累加平均和频域数据累加平均两种不同的数据处理方案。以稀释在载气氩气中0.5%的羰基硫(OCS)分子为标准样品,对以上方案在差频200 MHz和0.4 MHz处分别进行了实验验证。研究结果表明,外差式检测灵敏度高于零差式检测,但设备成本更高;频域数据累加平均较时域数据累加平均的信噪比好,但数据处理量大,数据处理耗时较长。

As one of the most important laboratory tools for the research of quantum chemistry and astrochemistry,a microwave spectrometer can make accurate measurement of molecular spectra arising from rotational transitions.Fourier transform microwave(FTMW)spectrometers,in many cases,require down-conversion mixing plans by using mixers to bring the molecular signal down for A/D sampling,and multiple signal averaging to improve the signal/noise ratio as well.Two detection schemes,homodyne and heterodyne detection modes,were proposed in this project along with two data processing plans,averaging in the time domain and averaging in the frequency domain,to develop efficient,sensitive and accurate detection solutions for molecular rotational spectroscopy.With 0.5%OCS gas in argon as standard sample,verification experiment was conducted with frequency difference between LO and cavity resonance at 200 MHz and 0.4 MHz.The experimental results indicate that,the heterodyne detection method can provide better sensitivity than the homodyne scheme,but with increased cost for more devices,while the time-domain averaging can obtain much better signal-to-noise ratio than frequency-domain averaging,but with stronger data processing capacity and more time required.