Tektronix SPECMON3 Manual page 53

General Signal Viewing
Figure 3
The traditional swept spectrum analyzer display, Figure 2, uses line traces that can show only one level for
each frequency point, representing the largest, the smallest or the average power. After many sweeps, the
Max Hold trace shows a rough envelope of the stronger laptop signal. +Peak detection was selected for the
other trace in an attempt to capture the weaker but more frequent AP signal, but the bursts are very brief,
so the likelihood of seeing one in any particular sweep is small. It will also take a long time to statistically
capture the entire spectrum of a bursted signal due to the architecture of the swept spectrum analysis.
The DPX display, Figure 3, reveals much more insight on the same signal. Since it is a bitmap image
instead of a line trace, you can distinguish many different signals occurring within each update period
and/or different version of the same signal varying over time. The heavy band running straight across the
lower third of the graph is the noise background when neither the laptop nor the AP is transmitting. The red
lump of energy in the middle is the ON shape of the AP signal. Finally, the more delicate spectrum above
the others is the laptop transmissions. In the color scheme used for this demonstration ("Temperature"),
the hot red color indicates a signal that is much more frequent than signals shown in cooler colors. The
laptop signal, in yellow, green and blue, has higher amplitude but doesn't occur nearly as often as the AP
transmissions because the laptop was downloading a file when this screen capture was taken.
How DPX Works
This section explains how DPX displays are created. The input RF signal is conditioned and
down-converted as usual for a signal analyzer, then digitized. The digitized data is sent through an FPGA
that computes very fast spectral transforms, and the resulting frequency-domain waveforms are rasterized
to create the bitmaps.
The DPX bitmap that you see on screen is composed of pixels representing x, y, and z values for
frequency, amplitude, and Density (instruments without Option 200 provide hit count as the z-axis value
in place of Density). A multi-stage process, shown in Figures 4a - 4d, creates this bitmap, starting with
analog-to-digital conversion of the input signal.
SPECMON3 & SPECMON6 Printable Help
DPX Primer
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