Measurement Technique of Low-Level and Low-Frequency Conductivity Fluctuations

In this paper the authors analyze a technique to measure both low-level and low-frequency conductivity fluctuations in noisy environments. This technique is based on a "lock-in detection" circuit with a feedback loop, containing an integrator and a modulator, to obtain a good signal-to-noise ratio and high sensitivity. Fluctuation frequency components lower than 0.01 Hz with sensitivity of 1 V per 100 ppm of mean value variation are achieved with an instrument based on this method.