Keysight N9038A - Noise Measurements Using Time Gating

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Concepts
Time Gating Concepts
Noise measurements using Time Gating
Time gating can be used to measure many types of signals. Noise and noise-like
signals are often a special case in spectrum analysis. With the history of gated
measurements, these signals are especially noteworthy.
The average detector is the best detector to use for measuring noise-like signals
because it uses all the available noise power all the time in its measurement. The
sample detector is also a good choice because it, too, is free from the peak biases of
the peak detector, normal and negative peak detectors.
When using the average or sample detector, noise density measurements using the
noise marker or band/interval density marker can be made without any consideration
of the use of gating--gated measurements work just as well as non-gated
measurements. Thus, the average detector is recommended for noise density
measurements.
Older analyzers only had the gated video version of gating available, and these only
worked with the peak detector, so the rest of this section will discuss the trade-offs
associated with trying to replicate these measurements with an X-Series analyzer.
Unlike older analyzers, X-Series analyzers can make competent measurements of
noise density using the noise marker with all detectors, not just those that are ideal
for noise measurements. Thus, X-Series analyzers can make noise density
measurements with peak detection, compensating for the extent to which peak
detection increases the average response of the analyzer to noise. When comparing a
gated video measurement using the noise marker between an X-Series and an older
analyzer where both use the peak detector, the X-Series answer will be
approximately correct, while the older analyzer will need a correction factor. That
correction factor is discussed in Keysight Technologies Application Note 1303,
Spectrum Analyzer Measurements and Noise, in the section on Peak-detected Noise
and TDMA ACP Measurements.
When making measurements of Band/Interval Power or Band/Interval Density, the
analyzer does not make compensations for peak detection. For best measurements
with these marker functions, average or sample detection should be used.