Tektronix 1503C, User Manual, Page: 121 / 128

Glossary
1503C MTDR User Manual
Glossary–5
time along the horizontal scale (across the display). The circuitry that captures and
holds the samples cannot instantly change from one voltage level to another. It might
take the circuit several samples to settle in at the new voltage after a rapid change
in the waveform. How efficiently the circuit moves from one sampled voltage level
to the next is called sampling efficiency. If the efficiency is too low, the waveforms
will be smoothed or rounded. If the efficiency is too high (above 100%), the circuit
will actually move beyond the new voltage level in a phenomenon known as
overshoot, which becomes an unwanted source of noise in the waveform.
Conductors all have some DC resistance to the flow of electrical energy through
them. The amount of resistance per unit length is usually nearly constant for a cable.
The energy lost overcoming this series resistance is called series loss. The series loss
must be compensated for when measuring the return loss or impedance mismatch
at the far end of long cables.
In a cable, a short circuit is a place where the signal conductor comes into electrical
contact with the return path or ground conductor. The electrical circuit is actually
shorter than was intended. Short circuits are caused by worn, leaky, or missing
insulation.
The change in accuracy of a standard or item of test equipment over an extended
period of time. Unless otherwise specified, the period of time is assumed to be the
calibration interval (might also apply to range, resolution, or precision as a function
of time). The term stability might also be used to denote changes resulting from
environmental influences, such as temperature, humidity, vibration, and shock.
An acronym for Time-Domain Reflectometer. These instruments are also called
cable radar. They send out pulses of energy and time the interval to reflections. If
the velocity of the energy through the cable is known, distances to faults in the cable
can be displayed or computed. Conversely, the speed that the energy travels through
a cable of known length can also be computed. The way in which the energy is
reflected and the amount of the energy reflected indicate the condition of the cable.
Electrical energy travels at the same speed as light in a vacuum. It travels slower than
that everywhere else. The speed that it travels in a cable is often expressed as the
relative velocity of propagation. This value is just a ration of the speed in the cable
to the speed of light (so it is always a number between 0 and 1). A velocity of
propagation value of 0.50 indicates that the electrical energy moves through the
cable at half the speed of light.
(see Noise)
Series Loss
Short Circuit
Stability
TDR
Velocity of Propagation
(Vp)
Waveform Averaging