Chapter 6 6-61
Front-Panel Key Reference
Peak Search
Marker Noise
Reads out the average noise level, referenced to a 1 Hz
noise power bandwidth. If the marker delta function is
on and the noise marker is activated and moved to
measure the noise floor, the marker readout will display
the signal-to-noise ratio.
NOTE You can use Delta and Marker Noise to indicate noise level relative to
sinusoidal signal, (signal to noise). However, be sure to activate the
delta marker before pressing
Marker Noise or the reference marker
value may be incorrect.
The noise marker averages 5% of the trace data values
(one-half a horizontal division), centered on the location
of the marker on the frequency or time scale. Marker
noise indicates noise power density per hertz or noise
voltage per root hertz depending upon the amplitude
units selected. The number of sweep points is noted in
parenthesis to the right of the sweep time in the lower
right corner of the graticule. If the analyzer has a
firmware revision prior to A.04.00, 32 trace data values
are averaged, and the number of points per sweep is
fixed at 401. Note that the data values averaged will
not always be symmetrical with respect to the marker
position.
If the marker is positioned within 2.5% of the beginning
of the trace (one-quarter division), the trace data values
in the first half-division will be averaged. Similarly, if
the marker is positioned within 2.5% of the end of the
trace, the trace data values in the last half-division will
be averaged. If the analyzer has a firmware revision
prior to A.04.00, and the marker is positioned within 16
trace points of the beginning or end of the trace, the
first 32 or last 32 trace data points, respectively, will be
averaged.
NOTE Do not use Marker Noise to evaluate the displayed average noise level
of the analyzer relative to the specification. Read the displayed average
noise level directly from the display or use the normal marker. Refer to
Application Note 150 for a discussion of how noise is displayed on a
spectrum analyzer.
A nominal correction for equivalent noise bandwidth is
made by the firmware based on a nominal 3 dB
resolution bandwidth. The firmware assumes the noise
bandwidth is 1.12 times the resolution bandwidth. This
means the shape of the resolution bandwidth filters
causes the noise power to be overstated by 1.12 times.
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