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Operating Instructions
1–12
1502C MTDR User Manual
Test Preparations
Vp is the speed of a signal down the cable given as a percentage of the speed of
light in free space. It is sometimes expressed as a whole number (e.g., 66) or a
percentage (e.g., 66%). On the 1502C, it is the percentage expressed as a decimal
number (e.g., 66% = .66). If you do not know the velocity of propagation, you
can get a general idea from the following table, or use the Help with Cables
section of the Cable Information menu. You can also find the Vp with the
procedure that follows using a cable sample.
NOTE. If you do not know the Vp of your cable, it will not prevent you from
finding a fault in your cable. However, if the Vp is set wrong, the distance
readings will be affected.
All Vp settings should be set for the cable under test, not the supplied jumper
cable.
Table 1–2: Vp of Various Dielectric Types
Dielectric Probable Vp
Jelly Filled .64
Polyethylene (PIC, PE, or SPE) .66
PTFE (Teflon R) or TFE .70
Pulp Insulation .72
Foam or Cellular PE (FPE) .78
Semi-solid PE (SSPE) .84
Air (helical spacers) .98
1. Obtain a known length of cable of the exact type you wish to test. Attach the
cable to the CABLE connector on the front panel.
2. Pull POWER on.
3. Turn the DIST/DIV to an appropriate setting (e.g., if trying to find the Vp of
a three-foot cable, turn the DIST/DIV to 1 ft/div).
4. Turn the
n
o
POSITION control until the distance reading is the same as the
known length of this cable.
5. Turn the Vp controls until the cursor is resting on the rising portion of the
reflected pulse. The Vp controls of the instrument are now set to the Vp of
the cable.
The Importance of Vp
(Velocity of Propagation)
Finding an Unknown Vp