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A review of frequency synthesizer-based microwave chemical sensors for dielectric detection of organic liquids

This paper presents a review of recently reported chemical sensors for dielectric detection at microwave frequencies. The sensors are based on a sensing element exposed to materials under test (MUT) as a part of a VCO inside a frequency synthesizer loop. The permittivity of the MUT is translated into a change in the free running frequency of the VCO and the DC control voltage of the loop which is read out using an Analog-to-Digital converter for further processing. Two different on-board and on-chip prototypes are presented and compared.