Characteristics Of Response In Time Domain; Masking - Agilent Technologies ENA Series User Manual

Characteristics of Response in Time Domain

This section describes masking and the identification of the mismatch type, which are
important for analyzing the response in the time domain.

Masking

Masking is a phenomenon in which a mismatch at a location near the calibration surface
affects the response at the next mismatch location. This occurs because energy reflected by
a mismatch at the location nearest to the calibration surface does not reach the next
mismatch location. For example, when you measure the reflection of a cable that has 2
mismatch locations reflecting 50% of the input voltage, the first mismatch reflects 50% of
the measurement signal. The remaining 50% reaches the next mismatch and its 50%, which
is 25% of the entire measurement signal, is reflected. Therefore, in the response in the time
domain, the second mismatch looks smaller.
NOTE In this example, the transmission line is assumed to have no loss. However, because there
actually is loss, the signal is attenuated as the distance from the calibration surface
becomes greater.
Figure 9-11 shows an example of masking due to loss. It compares masking when a short
termination is directly connected to the calibration surface and masking when a 6-dB
attenuator is inserted. In either case, total reflection occurs at the short termination. In the
latter case, the signal is attenuated in both ways, and the return loss appears to be −12 dB.
Figure 9-11 Example of masking
Chapter 9
Analysis in Time Domain (Option 010)
Characteristics of Response in Time Domain
349