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%.