180 GHz Pulsed CMOS Transmitter for Molecular Sensing

Abstract—The performance of a CMOS transmitter designed for molecular sensing applications having an operational band- width of 180−190GHz and peak output power of 0.6mW (−2.22dBm) is evaluated with a series of spectroscopic-based experimental trials. Continuous wave frequency modulated ab- sorption experiments to probe the Doppler and sub-Doppler line- shape profiles of the water rotational transition at 183.310 GHz demonstrate the tuning finesse and phase-noise characteristics of the integrated circuited embedded phase lock loop used to generate coherent mm-wave radiation. A description of the pulse modulator designed into the CMOS circuitry allowing for implementation of sensitive emission-based Fourier transform detection schemes is provided with performance characterized for spectroscopically relevant pulse durations (40-500 ns). These results are accompanied by a spectral analysis of the transmitter pulse signal leakage where the total isolation is measured to be 22dB. The first emission-based molecular detections obtained with this source are presented along with a discussion of how this transmitter will be incorporated into a future resonant cavity enhanced in-situ molecular sensor. Introduction: TRANSMITTER and receiver integrated circuit (IC) el- ements fabricated with silicon process technologies (CMOS and SiGe) are proving to be a viable platform from which to construct a variety of different sensor systems that operate in millimeter to terahertz region of the electromagnetic spectrum [1]. One such class of instrumentation to adopt these emerging technologies are mm-wave gas analyzers. Similar to examples using traditional cascaded multiplier chain based sources [2]–[4], gas analyzers deploying IC elements operate on the principal of continuous wave frequency modulated ab- sorption with analyte samples confined to customized gas cells. Such systems leverage the narrow molecular cross sections and wide instruments bandwidths allowing for the analysis of complicated gas mixtures as demonstrated with recent expelled human breath studies [5], [6].The intrinsically small and low power consumption qualities of IC based mm-wave source and detection electronics also make them suited for development into field deployable sys- tems that meet the stringent size and power requirements nec- essary for space borne in-situ chemical sensing applications. A prototype 90-100 GHz resonant cavity enhanced Fourier trans- form system [7], [8] employing a pulsed CMOS transmitter and heterodyne receiver pair has demonstrated sensitivity of 3×1010 molecules/cm3 (10 ppm in 100 mTorr) for a sample of CH3CN. The pulsed Fourier detection scheme deployed by this instrument allows for the observation of transient events highlighted by work [9] using laser ablation techniques to vaporize nonvolatile salt (KCl and NaCl) samples which are then be detected with the CMOS-based spectrometer. The work presented here expands on this previous generation of in- strumentation by pushing pulsed modulated CMOS transmitter technology to G-Band frequencies. This transmitter will serve as the foundation for a future cavity enhanced sensor having an operational bandwidth of 180-190GHz that includes the strong water transition at 183.301 GHz desirable for planetary science applications.