4. Operating Instructions
4.3. Change Velocity of Propagation (Vp)
Vp is expressed as a fraction of the speed of light. See Section 7.4 Velocity of Propagation (VoP,
Vp) for a detailed description of Vp.
The current setting for Vp appears on the lower center of the screen. Press the M-FUNC button
until the top-center information indicator reads Vp. Turn the M-FUNCTION knob and the Vp
value will change accordingly. Measurements and traces are automatically updated on the display
to reflect the new Vp.
All measurements done by the CT100B are based on time and then converted to distance for
display. An accurate Vp is necessary for accurate cable length and Distance-to-Fault calculations.
For cables of a known type, the Vp can be approximated by using the nominal Vp for that cable
type. The CT100B has a built-in library of cable types and their Vp values (see Section 4.5). You
can also measure the Vp by testing a known length of cable that is of the same type as the one you
want to test. The method is described in Section 4.4.
4.3.1. Fine Vp Control
By default, the CT100B displays three decimal places for Vp; however, the CT100B can display six
decimal places for Vp when very precise distance measurements are needed.
1. Select the MENU → Settings → Meas. Settings menu.
2. Select Vp 3 Sig Figs from the menu to toggle the option to Vp 6 Sig Figs. The CT100B will
display six decimal places for the Vp readout on the screen when Vp 6 Sig Figs is shown on
the menu.
3. When the CT100B is in Vp 6 Sig Figs mode, a new function, Fine Adj. Vp, is added to the
M-FUNC button. When the M-FUNC button is set to Fine Adj. Vp, turning the
M-FUNCTION knob will change the Vp value of the last three significant digits. The Vp
value of the first three significant digits can be changed by setting the M-FUNC button to
the Coarse Adj. Vp function.
4.4. 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 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. The accuracy of the Vp is dictated by the accuracy of the
physical measurement. If the cables are terminated, they can be included in the measurement
to find a combined Vp. The physical length measurement should not include any adapters
necessary for connecting the cable to the CT100B.
2. Attach the test cable to the CT100B. For the example illustrated in this manual, the test
cable is 3.0 feet long.
42 CT100B TDR Cable Analyzers Operator’s Manual
To Next Page
Loading...