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SECTION 3: TDR FUNDAMENTALS
3.1 First Time Start-up
Before using your Model 1205CXA, there are several
setup options you can choose from. Select the Setup
menu control and select the desired default settings
for the instrument. The options you choose will remain
selected, even when the instrument is turned off. (See
Section 2.3.8 for setup options.)
3.2 Cable Connection
It is important to establish a quality connection to the
cable under test. The TDR sends a high frequency
signal that is not efficiently transmitted through poor
connections or inadequate test leads.
3.3 Cable Check
Do a quick check of the cable. Get as close to the
suspected fault as possible. Use common sense when
examining the area near to the suspected fault. For
example, if there is a new fence, that is probably where
the problem is located.
When testing a section of cable where different types
of cable
are spliced, use the independent cursors and the
correct VOP for each section of cable to yield the most
accurate reading.
3.4 Cable Impedance
Any time two metallic conductors are placed close
together, they form a transmission line which has a
characteristic impedance. A TDR tests for a change
in impedance which can be caused by cable damage,
faulty splices, water ingress, change in cable type,
improper installation and even manufacturing flaws.
The insulating material that keeps the conductors
separated is called the cable dielectric. The impedance
of the cable is determined by the conductor diameter,
the spacing of the conductors from one another, and
the type of dielectric or insulation used.
3.5 Velocity of Propagation (VOP)
Determine VOP:
The VOP number of a cable is deter-
mined by the dielectric material that separates the two
conductors.
In a coaxial cable, the foam separating the
center conductor and the outer sheath is the material
determining the VOP.
In twisted pair, the VOP number
is determined by the spacing between conductors and
the insulation that separates them.
