A Sensor Interface System for Measuring the Impedance (Cx, Rx) of Soil at a Signal Frequency of 20MHz

A 20 MHz impedance-measurement system for measuring the equivalent parallel capacitance C_x and the shunting resistance R_x in artificial soil is presented. For water-content measurements in artificial soil, with a special two-electrode probe, the C_x values vary from 1 pF up to 30 pF. These small capacitances have to be measured in the presence of a shunting resistor R_x, with values ranging from 22 Omega up to 1 kOmega. The measurements have been performed with a sinusoidal signal with a frequency of 20 MHz. Based on an existing principle an improved interface has been developed, which enables accurate measurement of both the capacitive component C_x and its shunting resistance R_x. The high performance of the improved system has been obtained by implementing two direct digital synthesizers (DDS), which are used to reduce the nonlinearity in both the amplitude-ratio measurement and the phase measurement. Also so-called open/short compensation has been applied for compensating the parasitics in the measurement setup. Experimental results show that for C_x with a full-scale range of 30 pF the relative error amounts to 1% and 1.5% for shunting resistors of 820 Omega and 47 Omega, respectively. For the R_x component, for a full-scale range of 1 kOmega, the error is less than 1%.