67
TDR. Some signal may remain on the cable when measuring close to the
amplifier, but faults will still be visible. The quasi-square wave power (60
or 90VAC) must be removed from CATV cable before using the E20/20
TDR.
Velocity Search and Sample a Cable
“Velocity Search” and “Sample a Cable” are similar features with different
applications:
Velocity Search is intended to find the correct velocity for an installed system
when the measurement being used is to the nearest foot or meter and may have
some estimated or known error attached. Examples: for an installed cable with
“As Built”, “Planned Installation”, or “Cable plat” drawings, or “Pole to Pole
distances, or measuring wheels are being used as the distance guidance. This
VF obtained by the TDR will also include VF variations as slow downs going
through untwists at splices and VF variations between different manufactures
cables used on the same run.
Sample a Cable is intended for using cable samples from a box or reel of cable,
generally about 10ft (2m) to 50ft (15.2m) in length and which can be measured to
the inch or centimeters with relative accuracy. This produces a more accurate
VF calculation by the TDR when that VF is used on the rest of the cable from the
same box, reel, or lot number. The result will be more accurate measurements.
This feature also samples the impedance of the cable at the mid-range between
the TDR and the cursor marking the end of the sample. If accepted, it will alter
the cable’s Z
0
slightly.
Based on the above information, use the better of these two methods to obtain
the most accurate VF.
Velocity Search
Velocity Search is a process entered via the Cable menu as shown in figure 3-
64.
Figure 3-64
It involves connecting to an installed cable or cable system, select VELOCITY
SRCH using the
►
key and follow the on-screen instructions to start. Basically,