AEA Technology 20TDR, Основное руководство

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12
Impedance Effects
Impedance (Z) is simply defined as the resistance a conductor offers to AC frequencies and
measured in Ohms. Also known as characteristic impedance it is designed in at the time of
manufacture. Figures 2-2 and 2-3 show the effect of an open or short has on the cable’s
impedance. A Step TDR’s trace will react per the display on the right. The reading will change
from the horizontal normal impedance reading to either the maximum Ohms for an open or down
to 0 Ohms for a short.
Figure 2-2
Figure 2-3
Velocity Factor
A cable’s velocity factor (VF) is also designed in at the time of manufacture. VF is expressed as
a fraction of the speed of light in a vacuum [c]. A VF of 0.677c indicates that a transmitted wave
will travel along that cable at 67.7% the speed of light in a vacuum.
TDR’s measure the delay time of the reflected signals. The time is divided in two because it is
always a round trip, then multiplied by the cable’s VF in feet or meters per second to obtain the
distance to a particular reflection.
Time X Velocity = Distance
2
Loop Resistance
Along with measuring the impedance of a cable, a Step TDR also measures a cable’s loop
resistance. The result is a slight upward slope over distance of the cable as the loop resistance is
added to the impedance reading. This phenomenon is referred to as “Dribble Up.”
More detailed information about impedance, velocity factor and loop resistance are contained in
the TDR Operating Manual and AEA Technology’s TDR application notes found on the CD
shipped with the TDR and at
www.aeatechnology.com .
Zo= 0
Ω
Zo=50
Ω
or 100
Ω
Short
Short
Open
Zo=50
Ω
or 100
Ω
Zo
=
Open