We present a millimeter-wave Fourier transform emission spectrometer whose design is based on the application of a direct digital synthesizer (DDS) up-converted into the Ku-band with subsequent frequency multiplication. The spectrometer covers the frequency range from 50 GHz to 110 GHz and from 150 GHz to 330 GHz. Owing to the fast frequency switching ability of the DDS in the spectrometer, the same radiation source is used both as a generator of short polarizing pulses and as a local oscillator for the heterodyne receiving system. Such a design provides intrinsically coherent reception that allows very long-term data averaging in the time domain, which improves considerably the maximum sensitivity of the spectrometer. The performances of the spectrometer including the data acquisition rate, the sensitivity, and the accuracy of line frequency measurements were tested on the rotational spectra of OCS, NH2CHO, and CH3CH2CN. We show that in the frequency range of 150-300 GHz, the maximum sensitivity of the spectrometer for a 10 min integration time is around 10-9 cm-1 (the minimal value of the absorption coefficient of detectable rotational transition) in the case of narrowband single frequency pulse excitation, and around 10-8 cm-1 in the case of broadband chirped-pulse excitation.

中文翻译：

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基于直接数字合成的毫米波发射光谱仪

我们提出了一种毫米波傅里叶变换发射光谱仪，其设计基于直接数字合成器 (DDS) 上变频到 Ku 波段并随后进行倍频的应用。该光谱仪覆盖了 50 GHz 至 110 GHz 和 150 GHz 至 330 GHz 的频率范围。由于光谱仪中 DDS 的快速频率切换能力，同一辐射源既用作短偏振脉冲的发生器，又用作外差接收系统的本地振荡器。这种设计提供了本质上的相干接收，允许在时域中进行非常长期的数据平均，从而大大提高了光谱仪的最大灵敏度。光谱仪的性能包括数据采集率、灵敏度、并在 OCS、NH2CHO 和 CH3CH2CN 的旋转光谱上测试了线频率测量的准确性。我们表明，在 150-300 GHz 的频率范围内，光谱仪在 10 分钟积分时间内的最大灵敏度约为 10-9 cm-1（可检测旋转跃迁的吸收系数的最小值）。窄带单频脉冲激励，在宽带啁啾脉冲激励的情况下约为 10-8 cm-1。