3. Operating Instructions
3.10.3. Find an Unknown Velocity of Propagation (Vp)
This method is used to find the Vp for cables under test where the Vp is unknown. It requires a
sample cable that is of the same type that can be measured physically.
1. Measure the physical length of the test cable. Make sure the test cable is of the same type
for which you need to find Vp. If there are any adapters used while connecting the cable,
include their length in the measurement, or set one of the cursors at the end of the
adapter before attaching the cable.
2. Attach the test cable to the CT100B. In this example, the test cable is 3.0 feet long.
3. Press the
AUTOFIT/HELP
button, then press the
AUTOFIT
menu item. The CT100B
will now show the beginning and the end of the test cable (Figure 3.6). The cable end, in
this case an open termination, has been found at 3.023 feet and the whole cable trace is
displayed on the screen.
Figure 3.6.
Screenshot showing AUTOFIT result.
4. Use the
HORIZONTAL SCALE
knob to zoom in on the end of the cable and get a more
accurate placement (Figure 3.7) of the active cursor at the beginning or “toe” of the open
fault at the end of the cable.
5. Press the
M-FUNC
button to set the
M-FUNCTION
to Vp, and
Adjust Vp
using the
M-FUNCTION
knob until the CT100B distance measurement to the active cursor equals
the physical measurement. The final Vp value is the true Vp value of the cable
(Figure 3.8). Note that in the
Measurement
→
Meas. Settings
menu there is an option to
set Vp to six significant digits.
3.10.4. Smooth Settings
The CT100B smoothing feature performs temporal averaging to decrease visible noise on the
live TDR trace and improve signal-to-noise ratio (SNR) for first derivative, S
11
return loss, and
other mathematical operations. When you acquire baseline TDR scans of cable assemblies
meant for archiving and future comparison use, it is recommended that at least a moderate level
of smoothing be used so that the vertical resolution of the instrument be used to the fullest
extent possible. The effect of smoothing is most apparent at very small vertical scale settings
(that is, when the trace is zoomed-in vertically). To change TDR trace smoothing:
CT100B TDR Cable Analyzers Operator’s Manual
29
