Agilent Spectrum Analysis Basics - Agilent Technologies

Peak (positive) detection
One way to insure that all sinusoids are reported at their true amplitudes is
to display the maximum value encountered in each bucket. This is the positive
peak detection mode, or peak. This is illustrated in Figure 2-22b. Peak is the
default mode offered on many spectrum analyzers because it ensures that
no sinusoid is missed, regardless of the ratio between resolution bandwidth
and bucket width. However, unlike sample mode, peak does not give a good
representation of random noise because it only displays the maximum value
in each bucket and ignores the true randomness of the noise. So spectrum
analyzers that use peak detection as their primary mode generally also offer
the sample mode as an alternative.
Negative peak detection
Negative peak detection displays the minimum value encountered in each
bucket. It is generally available in most spectrum analyzers, though it is not
used as often as other types of detection. Differentiating CW from impulsive
signals in EMC testing is one application where negative peak detection
is valuable. Later in this application note, we will see how negative peak
detection is also used in signal identification routines when using external
mixers for high frequency measurements.
Figure 2-23a. Normal mode
Figure 2-23b. Sample mode
Figure 2-23. Comparison of normal and sample display detection when measuring noise
9. rosenfell is not a person’s name but rather a
description of the algorithm that tests to see if the
signal rose and fell within the bucket represented
by a given data point. It is also sometimes written as
“rose’n’fell”.
10. Because of its usefulness in measuring noise, the
sample detector is usually used in “noise marker”
applications. Similarly, the measurement of channel
power and adjacent-channel power requires a
detector type that gives results unbiased by peak
detection. For analyzers without averaging detectors,
sample detection is the best choice.
Normal detection
To provide a better visual display of random noise than peak and yet avoid
the missed-signal problem of the sample mode, the normal detection mode
(informally known as rosenfell 9 ) is offered on many spectrum analyzers.
Should the signal both rise and fall, as determined by the positive peak and
negative peak detectors, then the algorithm classifies the signal as noise.
In that case, an odd-numbered data point displays the maximum value
encountered during its bucket. And an even-numbered data point displays
the minimum value encountered during its bucket. See Figure 2-25. Normal
and sample modes are compared in Figures 2-23a and 2-23b. 10
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