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. Y ou c an t hen view the f r equency 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 electric al air li ne a nd ter minati on. 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.