Electrical impedance measuring apparatus for providing separate

Abstract

An oscillator provides an output signal, of frequency f.sub.0, whose phase is shifted by substantially 45.degree. to obtain a test signal which is coupled by a first cable to a bridge circuit located in proximity to a sample box and interconnected with an active, center electrode of that sample box. In response to the test signal, the bridge circuit provides a bridge output signal whose frequency is identical to that of the test signal and whose phase, relative to that of the test signal, and whose amplitude are related to the electrical admittance of particulate material within the sample box. The oscillator output signal is also applied to a phase-shifting network which provides first, second, third and fourth signals respectively in-phase, 90.degree. out-of-phase, 180.degree. out-of-phase, and 270.degree. out-of-phase with the test signal. A switching circuit couples the first, second, third and fourth signals to its output in a predetermined sequence and at a frequency of f.sub..alpha., wherein f.sub.0 is much greater than f.sub..alpha., to develop a reference signal which is coupled by a second cable to a first input of a double-balanced mixer located in proximity to the bridge circuit. The bridge output signal is applied to a second input of the double-balanced mixer, whereby a time-multiplexed mixer output signal is provided which has four successive DC levels proportional to positive and negative values of the conductance and susceptance components of the bridge output signal. The mixer output signal is coupled via the second cable and temperature and bulk density compensation circuits to first, second, third and fourth sample and hold circuits. The first and third sample and hold circuits store successive DC levels of the mixer output signal corresponding to positive and negative values of the conductance component, and the second and fourth sample and hold circuits store successive DC levels of the mixer output signal corresponding to positive and negative values of the susceptance component. A first operational amplifer takes the absolute difference of the stored DC levels in the first and third sample and hold circuits to provide a first output signal proportional to the conductivity of the particulate material, and a second operational amplifier takes the absolute difference of the DC level stored in the second and fourth sample and hold circuits to provide a second output signal proportional to the dielectric coefficient of the particulate material.