CE4000 Cable Explorer TDR
User Manual
VOP
The velocity of propagation is the speed at which the signal travels on the cable under test. The time required for the television signals to reach the customers is normally not a concern, however some delay is present in the system, due primarily to the delay of the cable and filters in the network.
The VOP of a cable is the speed at which the signal travels divided by the speed of light and is expressed as a decimal or as
apercentage (i.e. .85 or 85%). Most hard line cable will have a VOP of .87 or 87% or better. Typical (newer) drop cable will have a VOP of 80% to 85%. Older drop cables may have VOPs as low as 70%.
Approximately 50 cables are pre-programmed into the CE4000 for use. The user may add, delete or edit this data table. (VOP specifications are available from the cable manufacturer’s data sheet.) As shown in the previous equation, TDR simply measures the time between the incidence and reflected pulse and calculates the distance to the fault. Obviously, the accuracy of the VOP has a direct relationship to the accuracy of the resultant distance measurements. A 1% error in the VOP will result in a 1% error in the distance measurement, plus any inaccuracy in the time measurement capability of the TDR. A cable’s VOP can also be measured if the cable’s length is known by using the same formula. This procedure is detailed in the Operation section of this manual.
PULSE WIDTH
The pulse is 1/2 of a sine wave (negative pulse, but displayed as positive). The pulse width is typically programmable, so that longer pulses can be used for longer ranges—pulses with more energy can travel longer distances. The cable under test exhibits the same loss characteristics on the incidence pulse as they do on the typical RF signals on the network. The longer the cable is, the higher the loss. Note that the reflected pulse has traveled down the cable and back, thus it has experienced twice the cable attenuation.
