Time extraction method for time domain reflectometry measurements

Abstract

Determination and monitoring of moisture content in the soil is a requirement in many agricultural and civil engineering applications. Time domain reflectometry (TDR) is a well-known technique for its accuracy and applicability in the field. A typical TDR system consists of three units: A signal generation unit, a signal acquisition and encoding/decoding unit and a signal processing unit. The signal generation unit sends the electromagnetic waves in the form of pulses on a transmission line (probe), inserted in the soil. Based on the traveling time of the wave along the probe and characteristics of the reflected waves the dielectric constant of the soil is derived. This moisture content of soil is related to the dielectric constant using Topp's equation. Several TDR waveform interpretation methods have been reported. Though, many reported methods process the entire cycle of the TDR signal, the useful information is only available in 10-20% of the signal period, called the region of interest (ROI). Some of the methods extract this ROI manually. Some existing method describe the results by analyzing a single pulse of the TDR signal. This can lead to erroneous results as the single pulse may have been corrupted by either internal or external noise or by the jitter of the sampling clock. This paper presents a TDR waveform interpretation method. In this method, the ROI of 20 cycles is calculated automatically and averaged with the proper averaging technique. To study the effect of non-linearities added by the system on the TDR signal we have modeled the signal acquisition and encoding/decoding unit. We have also presented an error detection technique to detect the corrupted regions of a captured signal. The error detection technique is able to detect error level as low as 0.2% in the signal. The model has been tested with real TDR signals transmitted through air and water and then captured on a sampling oscilloscope, with different jitter levels and different number of bits in DAC. The TDR waveform interpretation method has been tested successfully with 5 different materials.