3- 33
Making Time Domain Measurements
Resolution
For example, a cable with a PTFE dielectric (0.7 relative velocity factor), measured under the conditions
stated above, has a fault location measurement response resolution of 0.45 centimeters. This is the
maximum fault location response resolution. Factors such as reduced frequency span, greater frequency
domain data windowing, and a large discontinuity shadowing the response of a smaller discontinuity, all act
to degrade the effective response resolution.
Figure 3-24 illustrates the effects of response resolution. The solid line shows the actual reflection
measurement of two approximately equal discontinuities (the input and output of an SMA barrel). The
dashed line shows the approximate effect of each discontinuity, if they could be measured separately.
Figure 3-24 Response Resolution
While increasing the frequency span increases the response resolution, keep the following points in mind:
• The time domain response noise floor is directly related to the frequency domain data noise floor.
Because of this, if the frequency domain data points are taken at or below the measurement noise floor,
the time domain measurement noise floor is degraded.
• The time domain measurement is an average of the response over the frequency range of the
measurement. If the frequency domain data is measured out-of-band, the time domain measurement is
also the out-of-band response.
You may (with these limitations in mind) choose to use a frequency span that is wider than the test device
bandwidth to achieve better resolution.
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