Tektronix 1503C, User Manual, Page: 118 / 128

Glossary
Glossary–2
1503C MTDR User Manual
Any substance that will readily allow electricity to flow through it. Good conductors
are metals such as silver, copper, gold, aluminum, and zinc (in that order).
dB is an abbreviation for decibel. Decibels are a method of expressing power or
voltage ratios. The decibel scale is logarithmic. It is often used to express the
efficiency of power distribution systems when the ratio consists of the energy put
into the system divided by the energy delivered (or is some cases, lost) by the
system. Our instrument measures return loss. The formula for decibels is: dB = 2–
log (Vi/Vl) where Vi is the voltage of the incident pulse, Vl is the voltage reflected
back by the load, and log is the decimal-based logarithmic function. The dB vertical
scale on our instrument refers to the amount of voltage gain (amplification) the
instrument applies to the signal before displaying it. For example, when the
instrument is amplifying the voltage by one hundred, the dB scale would read 40
dB, which is 20 log 100.
Direct current is a method of delivering electrical energy by maintaining a constant
flow of electrons in one direction. Even circuits designed to generate only AC often
have a DC component.
(see Insulation)
A mathematical term that refers to the set of numbers that can be put into a function
(the set of numbers that comes out of the function is called the “range”). A
time-domain instrument performs its function by measuring time.
The total opposition to the flow of electrical energy is a cable or circuit. Impedance
is made partly of resistance (frequency independent) and partly of reactance
(frequency dependent). Although impedance is expressed in units of Ohms, it must
not be confused with the simple resistance that only applies to DC signals.
Technically, impedance is a function of the frequency of the electrical signal, so it
should be specified at a frequency. As a practical matter, the impedance of most
cables changes very little over the range of frequencies they are designed for.
A point in a cable or system where the incident electrical energy is redistributed into
absorbed, reflected, and/or transmitted electrical energy. The transmitted electrical
energy after the mismatch is less than the incident electrical energy.
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.
(see Reactance)
Conductor
dB
DC
Dielectric
Domain
Impedance
Impedance Mismatch
Incident Pulse
Inductance