HP 8753E Time Domain Concepts Masking, Transmission Measurements Using Low Pass Impulse Mode

THRU LINE

FIBER

OPTIC

CABLE

(a) Comparing	Transmission	(b) Measuring Pulse Dispersion
Paths through a	Power Divider	on a 1.5 km 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