3-26
Making Time Domain Measurements
Masking
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 (=0.50). However, the second discontinuity appears as a 37.5%
reflection (=0.375) because only some the incident voltage reached the second discontinuity, and some of
that reflected energy is reflected off the first discontinuity as it returns. For two discrete discontinuities, the
apparent reflection of the second discontinuity is appears as approximately
, where is
the apparent reflection of the second discontinuity, is the reflection of the first discontinuity, and is
the reflection of the second discontinuity.
NOTE This example assumes a lossless 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 3-21a. When the short circuit is placed at the end of the 3 dB attenuator, the return loss
is 6 dB, as shown in Figure 3-21b. 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.
Figure 3-21 Masking Example
To Next Page
Loading...
Need help?
General
