TDR Fundamentals
Instrument Configuration
9-20
Instrument Configuration
In the proceeding sections little consideration was given to the effects of the
configuration of the oscilloscope and step generator on the measurement. Now
lets examine this important part of the TDR measurement.
There are several different architectures for accomplishing a TDR
measurement. They are:
• Terminated step generator and through-line sampler.
• Terminated sampler and through-line step generator.
• Terminated sampler, terminated step generator, and power splitter.
Traditionally, TDR systems have used the terminated step generator and
through-line sampler architecture shown in Figure 9-18.
Figure 9-18
The step generator was implemented using a tunnel diode. Because a tunnel
diode is a low impedance device, it lends itself to a terminated configuration.
The major drawback of this architecture is that small reflections from the
terminated step generator are measured directly by the through-line sampler.
In the HP 54750 Series TDR systems, the terminated sampler and through-line
step generator architecture in Figure 9-19 is used. In this case step generation
is accomplished using a switched current source driven from a step recovery
diode. This configuration is advantageous because small reflections from the
terminated sampler propagate back to the through-line step generator where
only a small portion of the already small reflection is sent back to the terminated
sampler and measured. None of the reflections from the step generator or the
sampler are measured directly, thus improving the system performance.
THRU-LINE
SAMPLER
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