Tektronix RSA6100A Series, Help Manual

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General Signal Viewing > DPX Spectrum DPX Primer
To adjust the box size, a mouse is the easiest way to drag the sides and corners of the rectangle. For precise
settings, use the knob, arrow keys, or keyboard to adjust frequency and amplitude values for the rectangle.
These controls are located in the right half of the Density tab in the control panel.
Persistence
Previous sections of this topic have assumed that persistence was not applied to the DPX bitmap. Without
persistence, hit counts in the grid are cleared after each frame update. Now we will describe how
persistence modi
fi es this behavior, starting with in fi nite persistence because it is simpler than variable
persistence.
Hit counts are not cleared between frames if in fi nite persistence is enabled. When the instrument is set
up for continuous acquisitions, hits keep collecting until you stop acquisitions or click the Clear button
above the DPX display. Software keeps track of the total number of waveforms computed during the
entire collection period. Density equals the total number of hits to a cell divided by the total number of
waveforms.
Variable persistence is trickier. A single-occurrence signal shown in the bitmap does not disappear
suddenly upon the next frame update, nor does it linger forever. It fades gradually away. The user sets
a time constant for the Dot Persistence control which determines how long it takes for signals to fade.
Fading is accomplished by reducing the hit count in every cell, after each frame update, by a factor based
on the persistence time constant. The longer the time constant, the less the hit counts are reduced.
Figure 26. Example of fast transient discovery with and without variable persistence turned on. In the display on the
left, with variable persistence of 10 seconds, the occasional sub-second transient that spikes up above the normal
signals is held in the display rather than disappearing as soon as the signal goes away. The display on the right, with
persistence turned off, requires watching the display continually to see the brief signal.
Not only are single-occurrence signals allowed to remain in the display for awhile by variable persistence,
additional hits keep piling on. The result is that cell values are no longer pure hit counts; they include
counts due to new hits from waveforms plus proportionally reduced counts from prior frames. As part
of translating hit counts into density values, a new software algorithm uses a
fi nite-series equation to
discriminate between the effects of persistence and the arrival of new hits. The in fl ationary effects of
RSA6100A Series Real-Time Spectrum Analyzers Help
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