Construction and preliminary results of a Ku-band chirped-pulse microwave Fourier transform spectrometer

The chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy technique is an efficient tool for the rapid measurement of broadband rotational spectra. Fast, state-of-the-art arbitrary waveform generators (AWG) and oscilloscopes are usually required for CP-FTMW spectrometers to induce and detect the free induction decay (FID) in a gas of molecules. Alternatively, the CP-FTMW spectrometer currently developed at UCLouvain makes use of a 2.5 GSa/s direct digital synthesizer in place of an AWG. It also leverages heterodyne detection to allow the acquisition of the FIDs with a 1 GSa/s oscilloscope. A quadrature demodulation technique is implemented to discriminate between the lower and upper sidebands of the probed frequency range. A computer program was developed to automate the scan of the Ku-band (12−18 GHz) and remove the spurious content from the spectra. Methanol spectra have been measured in the Ku-band to assess the performances of the spectrometer. The methanol sample was first studied at room temperature in a waveguide cell. The 12−18 GHz frequency range was covered by concatenation of multiple 500 MHz sub-spectra, all acquired and averaged 105 times in ∼ 13 minutes. In this configuration, the acquisition rate, precision and sensitivity of the instrument were evaluated. We fitted the FID in the time domain and extracted the resonant frequency and the T2 coherence time. First measurements performed in a supersonic expansion will also be presented.