FPGA-Based Broadband Measurement of Rotational Transitions With LO-Hopping and Quadrature-Detection Techniques

Microwave spectroscopy is crucial to chemical sensing, deep-space molecule hunting, as well as quantum mechanical studies. In order to make broadband measurement of rotational transitions convenient and affordable for most laboratories, a proof-of-concept miniaturized microwave spectrometer was designed and developed based on a common field-programmable gate array (FPGA) platform to mainly integrate a fast phase-locked microwave synthesizer board for local oscillator (LO) hopping, a direct digital synthesizing board for chirp pulsing and a homodyne quadrature-detection system for sideband identification, thus fulfilling the broadband and image rejection functions. Rotational spectra of three molecules, cyanoacetylene (HCCCN), dibenzofuran, and tert-butyl iodide, were measured during the instrumentation of this system-on-chip (SOC) design. The resulting prototype exhibited a single scan of its full bandwidth between 1.7 and 4.7 GHz by five local oscillator hops under one gas jet, with measurement accuracy of about 30 kHz and linewidth of about 250 kHz, comparable to many modern broadband designs but with much simpler and more compact apparatus structure. Although the operating range of the current version can only cover the S-band, this FPGA-based integration idea can be readily extended to much higher frequency and might lead to a trend-setting perspective for both instrumentation and related scientific research.