11.3. TDR ACCURACY
TDR accuracy depends on two main components: equipment-related issues and the test
system. The equipment issues include items such as step generator and sampler
imperfections that are controlled by the TDR manufacturer. The test system refers to the
measurement setup that incorporates cabling, probes, and type of DUT to be tested. Launch
parasitics, cable losses, multiple reflections, and interface transmission loss are all factors
that can affect TDR accuracy.
For the purposes of this section, only the test system is covered, as the end user has minor
control over the equipment's accuracy factors. In most cases the test system is the most
critical factor in taking accurate TDR measurements. As discussed earlier, the TDR extracts
impedance information based on the voltage response in time. Under ideal conditions, the
response is flat during the time of interest. In the real lab environment, the response may not
be so flat and could therefore affect the accuracy of the measurement. These nonideal
effects along the TDR response are referred to as aberrations.
In some cases, aberrations can be a significant percentage of the total response.
Aberrations due to the preshoot and foot of the response can be predictable. Predictable
responses can use processing techniques with software to account for these effects.
Unpredictable responses due to ringing and multiple reflections are not accountable. An
example where the response is predictable is illustrated in
Figure 11.14
. TDR pulse
response exhibits the foot, preshoot, and settling effects. These characteristics are also
replicated in the reflected portion of the response. Therefore, since the incident response
characteristics will be replicated in the reflected response, the possibility of utilizing software
or digital signal processing within the TDR could be used to help eliminate some
inaccuracies due to the predictable incident characteristics.
Figure 11.14: Example of how the incident pulse characteristics will be replicated.
11.3.1. Launch Parasitics
One cause of unpredictable response characteristics is launch parasitics. Launch parasitics
cause aberrations in the step response that can be unpredictable and therefore may not be
repeatable. The goal of launching a fast edge while maintaining signal integrity is largely
affected by reliable physical connection to the DUT, probe ground-loop inductance, and
structure crosstalk. Ground-loop inductance effects due to the probe type used as an
Summary :
TDR pulse response exhibits the foot, preshoot, and settling effects. Therefore, since the incident response characteristics will be replicated in the reflected response, the possibility of utilizing software or digital signal processing within the TDR could be used to help eliminate some inaccuracies due to the predictable incident characteristics. Launch Parasitics One cause of unpredictable response characteristics is launch parasitics.
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