Agilent Technologies 8719D, Руководство пользователя

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(a) Transmission
THRU LINE
FIBER
OPTIC
CABLE
Figure 6-69. Transmission Measurements Using Low Pass Impulse Mode
Time Domain Concepts
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
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 for the discontinuity
However, the second discontinuity appears as a 25% reflection 0.25) 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
attenuator and a short circuit. The impulse response (log magnitude format) of the short circuit
alone is a return loss of 0 as shown in Figure When the short circuit is placed at
the end of the 3 attenuator, the return loss is -6 as shown in Figure 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