Tektronix RSA6100A Series, Help Manual

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General Signal Viewing > DPX Spectrum DPX Primer
persistence on cell counts are removed, so density readings represent the true ratio of actual hits to possible
hits over the persistence interval.
The density computation for variable persistence is a very good estimate of true signal density, with errors
of less than 0.01%. For exact density measurements, use either no persistence or in
fi nite persistence.
Another subtlety of persistence is its smoothing effect on the density measurement of intermittent signals.
Consider a pulse that is on for 10 ms and off for 90 ms of each 100-ms cycle. We’ll make the simplifying
assumption that the pulse ON time always falls entirely within a single DPX frame update (50 ms). If
persistence is not applied, the density measurement is computed on each individual frame. The results
will be 20% for each frame containing the ON time and 0% for the other frames. If in
fi nite persistence
is enabled, however, the density measurement will settle to 10% after the second frame, and remain at
this value for as long as the pulsing continues. With persistence, the density is effectively computed
over many frames.
Persistence Effects on Density
Persistence does not alter colors in a density-based bitmap. Its effect is to extend the amount of time over
which densities are calculated, leaving signal events visible for the persistence duration.
Before the introduction of density measurements and extra-long hit counters, persistence caused colors
to “bloom”, becoming more and more intense over time as the hit counts increased. Longer persistence
intervals caused increased blooming, turning crisp signals into fat red stripes. When hit counts are
converted to density values (requires Option 200), the display is not subject to this effect. As long as the
input signals maintain reasonably stable repetition rates and duty ratios, their density values will also
remain stable despite ever-increasing hit counts in the underlying grid cells.
If you are accustomed to the original hit-count-based persistence displays, it may seem counterintuitive
that repeating signals in a density-based bitmap will not get brighter and redder over time with in
fi nite
persistence. A quick review of the density algorithm explains why: the hit count is divided by the total
number of waveforms over the persistence interval. For example, if a signal occupies a pixel 50% of the
time over a period of 15 minutes, the density reading will be 50% throughout the entire 15 minutes,
though the underlying hit count is steadily increasing.
Z-Axis Resolution
Another factor that can cause color bloom is over fl ow of the hit counters. If a pixel could only count up to
1000 hits, its density and color values would clip at 100% after just 1000 hits, even if waveform points
continue to arrive in the same pixel location. With waveform points being written to the bitmap at rates
approaching 300k/sec, counts add up really fast for highly-repetitive signals. Deeper counters permit
higher hit counts, so over
fl ow happens much later, as shown in Table 3.
Table 4: Comparison of DPX z-axis resolution and its effect on saturation.
RSA6100A Series Standard RSA6100A Series Option 200
Hit Count 16-bit integer 36-bit custom fl oat (equivalent to
33-bit integer)
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RSA6100A Series Real-Time Spectrum Analyzers Help