3-12
Making Time Domain Measurements
Time Domain Bandpass Mode
Time Domain Bandpass Mode
This mode is called bandpass because it works with band-limited devices. Traditional TDR requires that the
test device be able to operate down to dc. Using bandpass mode, there are no restrictions on the
measurement frequency range. Bandpass mode characterizes the test device impulse response.
Adjusting the Relative Velocity Factor
A marker provides both the two-way time and the two-way electrical length (or distance) to a discontinuity.
The distance displayed is based on the assumption that the signal travels at the speed of light. The signal
travels slower than the speed of light in most media (e.g. coax cables). This slower velocity (relative to light)
can be compensated for by adjusting the analyzer relative velocity factor. To determine the physical length,
rather than the electrical length, change the velocity factor to that of the medium under test:
1. Press .
2. Enter a velocity factor between 0 and 1.0 (1.0 corresponds to the speed of light in a vacuum). Most
cables have a velocity factor of 0.66 (polyethylene dielectrics) or 0.70 (PTFE dielectrics).
NOTE To cause the markers to read the actual one-way distance to a discontinuity, rather than the
two-way distance, enter one-half the actual velocity factor.
Reflection Measurements Using Bandpass Mode
The bandpass mode can transform reflection measurements to the time domain. Figure 3-10 (left) shows a
typical frequency response reflection measurement of two sections of cable. Figure 3-10 (right) shows the
same two sections of cable in the time domain using the bandpass mode.
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