A SiGe BiCMOS dielectric sensor utilizing an open-ended microstrip line in a 28 GHz colpitts oscillator

In this paper, a microwave dielectric sensor is presented using an open-ended microstrip line. It is fabricated in a standard 0.25 μm SiGe BiCMOS process (f_T/f_max = 180/220 GHz). A theoretical basis of open-ended microstrip line as dielectric sensor has been proposed and it is used as a permittivity dependent capacitor in the Colpitts oscillator. The sensor distinguishes permittivities ranging from 1 to 20 in the frequency range 26 to 29 GHz. With no material on top of the sensor, oscillation frequency settles at 28.72 GHz. Air, honey, epoxy resin (gray and orange) were used to verify and calibrate the sensor experimentally. In a further step, the sensor has been used to characterize water fraction in supersaturated sugar solution (honey). A sensitivity of 250 MHz/Unit permittivity has been shown. It spans an area of 0.6 mm² and consumes 4.5 mA from a 3.3 V supply.