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(a) Comparing Transmission
(b) Measuring Pulse Dispersion
Paths through a Power Divider
on a
1.5 km
Fiber Optic Cable
THRU LINE
FIBER
OPTIC
CABLE
Figure 6-69. Transmission Measurements Using Low Pass Impulse Mode
Time Domain Concepts
Masking
Masking
occurs when a discontinuity (fault) closest to the reference plane affects the response
of each subsequent discontinuity. This happens because the energy reflected from the first
discontinuity never reaches subsequent discontinuities. For example, if a transmission line has
two discontinuities that each reflect 50% of the incident voltage, the time domain response
(real format) shows the correct reflection coefficient for the
first
discontinuity (p= 0.50).
However, the second discontinuity appears as a 25% reflection
(p=O.25)
because only half the
incident voltage reached the second discontinuity.
Note
This example assumes a loss-less transmission line. Real transmission lines, with
non-zero loss, attenuate signals as a function of the distance from the reference
plane.
As an example of masking due to line loss, consider the time domain response of a 3
dB
attenuator and a short circuit. The impulse response (log magnitude format) of the short circuit
alone is a return loss of 0
dB,
as shown in Figure
6-70a.
When the short circuit is placed at
the end of the 3
dB
attenuator, the return loss is -6
dR,
as shown in Figure
6-70b.
This value
actually represents the forward and return path loss through the attenuator, and illustrates
how a lossy network can affect the responses that follow it.
Application and Operation Concepts
6-135