Gas spectroscopy with 245 GHz circuits in SiGe BiCMOS and Frac-N PLL for frequency ramps

A compact gas spectroscopy system is presented, which contains a 245 GHz transmitter (TX) and a subharmonic receiver (RX) in SiGe BiCMOS technology, as well as a gas absorption cell. The local oscillators (LOs) of the RX and the TX are controlled by two external Fractional-N phase-locked loops (Frac-N PLLs) allowing fast frequency ramps with superimposed frequency shift keying (FSK) or analog frequency modulation. The sensitivity of this spectroscopic system is evaluated by measuring the high-resolution absorption spectrum of gaseous methanol (CH3OH). Spectra of CH3OH with a high signal-to-noise ratio (SNR) are shown for the range 241–242 GHz.