Tektronix RSA6100A Series, Help Manual

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General Signal Viewing > DPX Spectrum DPX Primer
Figure 28. With variable persistence, a brief CW signal captured by DPX remains in the display for an adjustable
period of time before fading away.
DPX Density Trigger
The standard DPX display shows you a clear picture of transients and other hard-to- fi nd signals. The
version of DPX in the RSA6100A Option 200 goes well beyond helping you discover these dif fi cult to fi nd
signals by actually triggering on their appearance to capture them into acquisition memory for in-depth
analysis. If you can see it in the DPX bitmap, you can trigger on it.
Other trigger methods can detect signals that exceed an amplitude threshold, or even a sophisticated
amplitude-vs-frequency mask, but they can’t
fi nd a signal at a particular frequency if another signal of
higher amplitude is sometimes present at that same frequency. The Runt trigger addresses some of these
signal-under-signal cases, but not all. As shown in Figure 29, the DPX Density trigger can discriminate
signals within a precise amplitude-frequency range without the operator having to know any characteristics
of the target signal besides where it might show up in the DPX Spectrum graph.
Figure 29. Example of Density Trigger. Left: A free-run DPX spectrum display showing pulses with varying frequency.
Occasionally, a short pulse in the middle appears for a split instant, but it is hard to capture it with just a Run/Stop
button. Right: The triggered DPX displays shows the low-amplitude pulse that was not apparent in the untriggered
display. The analyzer was set to trigger whenever the average density in the user-drawn box measured 50% or higher.
The DPX Density trigger uses the same screen-based measurement box as the DPX Density measurement.
While the target signal is absent, the density measurement characterizes the “normal” signals within the
box. When the target signal
fi nally appears, the density value increases. The trigger system monitors the
density measurement and activates a trigger whenever the density value exceeds the adjustable density
threshold. The only thinking you have to do is to set this threshold to a level somewhere between the
normal density readings and the density due to the trouble-making signal. However, the instrument
software can compute the threshold value automatically.
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RSA6100A Series Real-Time Spectrum Analyzers Help