Fluke DSP-2000 - Manual (Page 127)

Basic Cable Testing
Time Domain Reflectometry (TDR)
7
7-15
NVP Calibration
The NVP values specified for standard cables are included in the cable
specifications stored in the test tool. These values are accurate enough for most
length measurements. However, the actual NVP for one cable type can vary up to
20% between batches because of variations in the manufacturing process.
Therefore, if accurate length measurements are critical to your installation or
testing process, you should determine the actual NVP value for each spool of
cable. Determining the NVP value involves measuring a known length of cable
and adjusting the test tool’s length measurement to match the known length. As
you adjust the length measurement, the NVP value changes accordingly. The
calibration procedure is explained in “NVP Calibration” in Chapter 6.
Time Domain Reflectometry (TDR)
TDR is a measurement technique used to determine a cable’s length and
characteristic impedance and locate faults along the cable. TDR is sometimes
referred to as cable radar because it involves analyzing signal reflections in the
cable.
If a signal traveling through a cable encounters an abrupt change in the cable’s
impedance, some or all of the signal is reflected back to the source. The timing,
size, and polarity of the reflected signals indicate the location and nature of
impedance discontinuities in the cable.