“…The absorption line profiles have been recorded in the present study by the AIST terahertz spectrometer [2,26,27] using a backward-wave oscillator (BWO, IS-TOK OV-80, Russia) as a radiation source. The frequency of the BWO is controlled by a millimeter-wave (mmW) synthesizer (KVARZ, Russia) oscillating around 100 GHz as a local oscillator, with a phaselock-loop (PLL) using an intermediate frequency of 350 MHz generated by a harmonics mixer, which mixes the mmW synthesizer and the BWO radiations.…”