Campbell Scientific TDR100 Time Domain Reflectometry Systems - Manual (Page 37)

TDR100
9.2.1.1 Waveform Evaluation
Figure 9-1 shows a typical waveform and identifies key points. The waveform
before point 1 is the section of coaxial cable near the probe head with point 1
the transition from the 50 ohms of the cable to the impedance of the probe.
The change in reflection coefficient between points 1 and 2 is related to the
difference in impedance between the cable and the probe. Point 2 corresponds
to the rods exiting the probe head and point 3 is the end of the probe rods. The
apparent length relates to the difference between point 2 to point 3 and is
obtained by converting waveform data points to distance. The conversion uses
the number of waveform data points (250 for the waveform in Figure 9-1) and
the width of the waveform window as defined by PCTDR or datalogger
instruction parameters.
2
0 50 100 150 200 250
0.3
0.2
0.1
0
0.1
waveform data point
reflection coefficient (rho)
1
3
FIGURE 9-1. Typical TDR100 waveform showing key features with numbers 1, 2 and 3.
The algorithm to calculate apparent length uses both the waveform values and
the first derivative of the waveform to identify the beginning and end of the
probe. Beginning at the first data point, the algorithm searches for a value near
point 1. This rudimentary search result is used to begin a refined search for the
maximum derivative value which occurs between points 1 and 2. The
derivative value is used to define a tangent line through the point of maximum
derivative. The intersection of the tangent line and the waveform for the cable
near the probe defines the start of the probe. The probe offset is applied to
identify the waveform point associated with the rods exiting the probe body
which is near point 2. This method is more robust than simply finding point 2.
The algorithm then searches for the end of the probe rods which is near point
3.
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