Agilent Technologies 24A Guía de usuario descargar pdf (Pagina 206)

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3-26

Making Time Domain Measurements

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 ρ=0.50). However, the second discontinuity appears as a 25% reﬂection

ρ=0.25) because only half the incident voltage reached 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

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