Millimeter-wave emission spectrometer based on direct digital synthesis.

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.