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
(teflon 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.
Cal
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