3- 35
Making Time Domain Measurements
Gating
Gating
Gating provides the flexibility of selectively removing time domain responses. The remaining time domain
responses can then be transformed back to the frequency domain. For reflection (or fault location)
measurements, use this feature to remove the effects of unwanted discontinuities in the time domain. You
can then view the frequency response of the remaining discontinuities. In a transmission measurement, you
can remove the effects of multiple transmission paths.
Figure 3-26a shows the frequency response of an electrical airline and termination. Figure 3-26b shows the
response in the time domain. The discontinuity on the left is due to the input connector. The discontinuity on
the right is due to the termination. We want to remove the effect of the connector so that we can see the
frequency response of just the airline and termination.
Figure 3-26c shows the gate applied to the connector
discontinuity. Figure 3-26d shows the frequency response of the airline and termination, with the connector
"gated out."
Figure 3-26 Sequence of Steps in Gating Operation
Setting the Gate
Think of a gate as a bandpass filter in the time domain (see Figure 3-27). When the gate is on, responses
outside the gate are mathematically removed from the time domain trace. Enter the gate position as a start
and stop time (not frequency) or as a center and span time. The start and stop times are the bandpass filter
6 dB cutoff times. Gates can have a negative span, in which case the responses inside the gate are
mathematically removed. The gate's start and stop flags define the region where gating is on.
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