Impedance measurements with second-order harmonic oscillator for testing food sterility

In this paper, we present a simple interface circuit for measuring impedance based on a second-order harmonic oscillator. The circuit is intended for testing the sterility of aseptically packed food products, where it must measure the conductivity changes of the packaged food in a nondestructive way. The measured impedance is modeled as a low-ohmic resistor (representing the conductivity of the food) in series with a capacitor (the walls of the food container). The oscillator is built with fast current-feedback operational amplifiers (CFOA). To measure the resistive component of the impedance, an AC-to-DC converter is realized with a wide frequency range analog multiplier. The presented experimental results prove that with this interface circuit an accurate measurement of the impedance components can be achieved in a frequency range up to 10 MHz. The range of the resistive component is from a few ohms up to 200 ohms; the range of the capacitive component is from 50 pF up to 300 pF.