Glossary
1502C MTDR User Manual Glossary–3
Impedance Mismatch
A point in a cable or system where the incident electrical energy is redistrib-
uted into absorbed, reflected, and/or transmitted electrical energy. The
transmitted electrical energy after the mismatch is less than the incident
electrical energy.
Incident Pulse
The pulse of electrical energy sent out by the TDR. The waveform shown by
the TDR consists of this pulse and the reflections of it coming back from the
cable or circuit being tested.
Inductance
(see Reactance)
Insulation
A protective coating on an electrical conductor that will not readily allow
electrical energy to flow away from the conductive part of the cable or
circuit. Insulation is also called dielectric. The kind of dielectric used in a
cable determines how fast electricity can travel through the cable (see
Velocity of Propagation).
Jitter
The short term error or uncertainty in the clock (timebase) of a TDR. If the
timing from sample to sample is not exact, the waveform will appear to
move back and forth rapidly.
LCD
An acronym for Liquid Crystal Display. It is the kind of display used on this
instrument, so the terms display and LCD are often used interchangeably.
Millirho
rho (r) is the reflection coefficient of a cable or power delivery system. It is
the ratio of the voltage reflected back from the cable or circuit due to cable
faults or an impedance mismatch at the load, divided by the voltage applied
to the cable. Millirho are thousandths of one rho. Rho measurements are
often used to judge how well the cable is matched to the load at the other end
of the cable. If there is an open circuit in the cable, nearly all the energy will
be reflected back when a pulse is sent down the cable. The reflected voltage
will equal the incident pulse voltage and rho will be +1. If there is a short
circuit in the cable, nearly all the energy will be delivered back to the
instrument through the ground or return conductor instead of being sent to
the load. The polarity of the reflected pulse will be the opposite of the
incident pulse and rho will be –1. If there is no mismatch between the cable
and the load, almost no energy will be reflected back and rho will be 0. In
general, a load or fault with higher impedance than the cable will return a rho
measurement of 0 to +1, and a load or fault with a lower impedance will
return a rho measurement of 0 to –1. The scale for rho measurements is
determined by the height of the incident pulse. A pulse two divisions high