Chapter 2 135
Making Measurements
Example 11: Time-Gated Measurement
Figure 2-67 Using Time-Gating to View Signal 2
Time-gating serves as a useful measurement tool for many different
types of signals. However, the signal must be repetitive and have a TTL
timing trigger signal available to synchronize the gate.
Making Noise Measurements Noise measurements made using a
gated measurement technique will vary from the value measured
without gating. A typical measurement made without gating uses
sample detection mode and has some errors caused by the
instrumentation. The errors come from the differences between the
filter shape of the resolution bandwidth filters compared with ideal
filters, and from the spectrum analyzer internal block diagram which
takes the logarithm of the noise first and than averages it.
A gated measurement is also affected by these considerations, and it
uses peak detection rather than sample detection. This adds additional
inaccuracy because the peak value of the noise during some time
interval will give a higher result than the average power (using sample
detection). The resulting value increases as the time interval increases,
because the probability of finding the statistically rarer larger peaks
increases. For very accurate noise measurements using the gated
function, the impact of these considerations must be calculated based
on the current spectrum analyzer settings.
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