A Microwave Permittivity Sensing System Based on a Hybrid Heterodyne Reflectometer Circuit

In this paper a microwave permittivity sensing system based on a super-heterodyne reflectometer is presented. A set of two high directivity directional couplers is utilized for the derivation of a reference and a reflection measurement channel from a 14 - 18 GHz stimulus. Integrated Gilbert-cell circuits were fabricated in a SiGe HBT technology and act as frequency converters for the high frequency measurement signals to an intermediate frequency of 1 GHz. A second mixer stage is implemented for further down-conversion to a second IF as low as 20 kHz. Low-cost analog-to-digital converters and a digital signal processor provide the measurement results on a serial interface. Permittivity measurements with a coaxial dielectric probe were performed on five different mixture ratios of ethanol and methanol.