Permittivity is an important property of dielectric materials. By measuring the
permittivity of a material, it is possible to obtain information about the material’s physical and
chemical properties, which are of great importance to many applications. In this study, a realtime
control system for a frequency-response (FR) permittivity sensor was developed. The core
of the hardware was a kitCON167 microcontroller (PHYTEC America, LLC), which controlled
and communicated with peripheral devices. The system consisted of circuits for waveform
generation, signal conditioning, signal processing, data acquisition, data display, data storage,
and temperature measurement. A C program was developed in the TASKING Embedded
Development Environment (EDE) to control the system.
The control system designed in this study embodied improvements over a previously
designed version in the following aspects: 1) it used a printed circuit board (PCB); 2) the
measurement frequency range was extended from 120 MHz to 400 MHz; 3) the resolution of
measured FR data was improved by using programmable gain amplifiers; 4) a data storage
module and a real-time temperature measurement module were added to the system; 5) an LCD
display and a keypad were added to the system to display the FR data with corresponding
frequencies and to allow users to enter commands.
Impedance transformation models for the sensor probe, the coaxial cable that connects
the control system with the sensor probe, and the signal processing circuit were studied in order
to acquire information on the permittivity of measured materials from measured FR data. Coaxial
cables of the same length terminated with different loads, including an open circuit, a short
circuit, a 50 resistor, and a 50 resistor paralleled by a capacitor, were tested. The results
indicated that the models were capable of predicting the impedances of these specific loads using
the FR data. Sensor probes with different sizes and coaxial cables with two different lengths
terminated with the same sensor probe were also tested. The results were discussed.
Additional tests for the gain and phase detector were conducted to compare FR data
measured by the gain and phase detector with those observed on an oscilloscope. The results
were discussed.
