Environmental Engineering Reference
In-Depth Information
Indeed, there is a wide availability of TDR units on the market: ranging from
low-cost portable units to high-performance benchtop instrument.
Most of the experimental results reported in this topic were obtained using
two TDR instruments with different characteristics, namely the Hyperlabs 1500
(HL1500) and/or the Tektronix TDR80E04.
The Hyperlabs 1500 is a low-cost portable TDR unit (see Fig. 3.5) [7]. The rise
time of the generated signal is approximately 200 ps, which corresponds to a fre-
quency bandwidth of around 1.7 GHz. The incident pulse amplitude is 250 mV and
the pulse duration is 14
s. The maximum number of acquisition points, for the se-
lected time window, is 2048. The HL1500 is a single-port instrument; the output has
a BNC connector and the output impedance is 50
μ
. The HL1500 can be controlled
through a computer. This TDR instrument supports multiplexer, thus allowing the
simultaneous connection of several probes to a single TDR unit. This feature is
particularly useful for practical purposes (e.g., for monitoring along the production
lines, for geotechnical analysis of soils, etc.), as it would lower the implementation
costs.
Ω
Fig. 3.5 Picture of the
HL1500 portable TDR in-
strument [7]
As for the Tektronix TDR80E04, it is a highly sophisticated TDR module that
is mounted on the Tektronix Digital Serial Analyzer DSA8200 (a benchtop instru-
ment) shown in Fig. 3.6. The TDR incident step signal has a rise-time of approxi-
mately 23.5 ps, thus the frequency content is about 15 GHz. The incident pulse am-
plitude is
s. The maximum num-
ber of acquisition points, for the selected time window, is 4000. The TDR80E04 has
two ports (as it also allows transmission measurements to be performed). The output
ports have 3.5 mm connectors.
These two specific instruments were chosen, because they are representative
of two categories of TDR instrumentation. The HL1500 is a compact and rugged
TDR unit, and this make it particularly useful for in situ applications. On the other
hand, the TDR80E04 is a high-performance instrument, and it is typically used in
laboratories.
±
250 mV and its duration is approximately 2.65
μ
 
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