OPERATION
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traces, even though they are meant to be a more instantaneous view of the spectrum, can also have a
higher perceived noise floor because they are also constructed using peaks. When you are looking at a
very narrow span, as long as you are not in Detail Zoom mode (see page 45), the tuner is able to sweep
much faster than the display screen can refresh so that each display update may contain data from
several sweeps – 150 sweeps or more at the narrowest span. (The sweep count indicator will display
the number of sweeps contained within each spectral display update.) Each screen update will only
contain peaks of all the sweeps that occurred between screen updates. As you expand the display and
begin looking at a wider spectrum, fewer sweeps occur between each refresh of the display; however, in
order to fit the entire spectrum on the display, each pixel will begin to represent a larger bandwidth – as
much as 30 MHz at the maximum span on a 24 GHz model. All of the points within this bandwidth are
peaked and only the highest peak is displayed. The combined effect of both of these phenomena cause
the perceived noise floor of the real time trace to be higher, not as high as the peak trace, but still
higher than the actual noise floor. A more accurate depiction of the noise floor level can be achieved
through using the average trace, because each trace, rather than being peaked, is averaged. To see the
actual noise floor, use the average trace, with Traces Averaged set to 1, and set the span to less than 10
MHz.
From the screenshot below, it is easy to see that the peak trace (in red) has the highest perceived noise
floor, then the real time trace (in yellow), and finally the average trace (in orange) displays the actual
noise floor.
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