A permittivity sensitive PLL based on a silicon-integrated capacitive sensor for microwave biosensing applications

This paper proposes a permittivity sensitive phase-locked loop (PLL) for the characterization of biologically and medically relevant materials and liquids in a frequency frange from 19.2 GHz to 20.8 GHz. It utilizes a custom design voltage controlled oscillator (VCO) with a tank-integrated capacitive sensor fabricated on a SiGe process. A change of the permittivity opposed to the sensor leads to a frequency shift which will be regulated by the PLL. Thus, the permittivity of a MUT is directly mapped to the resulting DC tuning voltage generated by the control loop. The sensor is configured and read-out by a microcontroller. The system is used to characterize solutions of ethanol-methanol mixtures with concentration differences of 5 percent around a mixture ratio of 50:50.