Specifications

152 Chapter 15
Concepts
Time Gating Concepts
Concepts
Noise Measurements Using Time Gating
Time gating can be used to measure many types of signals. However,
they must be repetitive and for ESA, have a TTL timing trigger signal
available to synchronize the gate. Noise is not a repetitive signal, so if
you need to use gating when measuring noise, you should understand
the impact on the measurement results.
To measure the power accurately of a noisy signal, or noise-like signal
with time gating, a sample or average detector should be used. Average
and sample detection is available for both ESA and PSA spectrum
analyzers when time gating is on.
If peak detection is used during a gated measurement the power
reading will be higher than if average detection is used. 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.
The equation below can be used to calculate a correction value for the
measured noise using peak detection. Subtract the correction from the
measured value.
where:
BW
i
is the impulse bandwidth
ESA is approximately 1.62
× resolution bandwidth, for
resolution bandwidths
≥1kHz.
PSA is approximately 1.5
× resolution bandwidth, for
resolution bandwidths
≤3 MHz.
τ is the time interval over which the peak detection
occurs and is equal to the sweep time/(number of sweep
points
− 1).
Refer to Agilent Technologies Application Note 1303, page 18, for more
details.
C
orrect
i
on
10
l
og
10
2
πτ
BW
i
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n
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=