A K-Band Complex Permittivity Sensor for Biomedical Applications in 130-nm SiGe BiCMOS

In this brief, an integrated K-band complex permittivity sensor for characterization of biomaterials is presented. The circuit enables determining both real and imaginary part of the permittivity of a material under test exposed to the sensor. We propose several enhancements of the classical capacitive sensing concept, which is based on an oscillator with a resonant tank and a capacitor realized as an integrated stub. To resolve the inherent trade-off of this method between sensitivity to variation of ε' and frequency tuning range of the oscillator, we propose using magnetic tuning of the resonant tank inductor. Additionally, the amplitude of oscillation gives an indication about the quality factor of the resonant tank, which can be translated into material losses, quantified by ε” or loss tangent. To guarantee the oscillator startup even for high loss materials, we propose using a feedback loop sensing the amplitude of the oscillation and controlling the bias current. Finally, we integrate a digitally tunable reflection type phase shifter to enhance the sensitivity. The chip is fabricated in a 130-nm SiGe BiCMOS technology. It consumes only 24 mA from a single 1.5-V supply and occupies a chip area of 1.8 mm
²
. The sensor has been calibrated using ethanol-water solutions in steps of 5% of concentration change.