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Figure 6-68. Transmission Measurements Using Low P ass Impulse Mode
Time Domain Concepts
Masking
Masking occurs when a discontinuity (fault) closest to the reference plane aects the response
of each subsequent discontinuity . This happens because the energy reected from the rst
discontinuity never reaches subsequent discontinuities . F or example , if a transmission line has
two discontinuities that each reect 50% of the incident voltage , the time domain response
(real format) shows the correct reection coecient for the rst discontinuity (=0.50).
However , the second discontinuity appears as a 25% reection (=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 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 6-69a. When the short circuit is placed at
the end of the 3 dB attenuator , the return loss is 06 dB , as shown in Figure 6-69b . This value
actually represents the forward and return path loss through the attenuator , and illustrates
how a lossy network can aect the responses that follow it.
Application and Operation Concepts 6-127