Agilent Technologies 54503A User's Guide download pdf (Page 135)

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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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