Tektronix SPECMON3 Manual page 63

General Signal Viewing
Guaranteed Capture in DPX Swept Spans
Probability of intercept (POI) for signals within a single segment, while DPX is dwelling in that segment,
is the same as for non-swept DPX operation (POI = 100% for events as brief as 10.3 microseconds).
But just as in traditional swept analyzers, during the time the acquisition is tuned to any one segment,
the analyzer is not monitoring signals in any of the other segments, so probability of capture in segments
other than the current one is zero. Because of the wide real-time bandwidth, the number of segments
needed to cover the span is much less than for swept analyzers, so the overall probability of intercept is
significantly better for DPX sweeps.
Another factor affecting POI is number of trace points. The bitmap is always 801 points wide, but the
line traces allow user selection for number of points. 801 is the default and the other choices are 2401,
4001, and 10401. Frequency transforms for traces containing more than 801 points take longer, and this
lower waveform update rate increases the minimum signal duration proportionally. This caution applies
for swept and non-swept operation. The trace length control is on the Prefs tab in the DPX control panel.
DPX Density Measurements
"Density" is a measure of the amount of time during a defined measurement period during which signals
are present within a particular area of the DPX Spectrum bitmap. A clean CW tone gives a 100% reading,
while a pulse that is on for one microsecond out of every millisecond reads 0.1%. This section describes
how density is computed from hit counts.
If we plot 41 more waveforms into the example grid we used previously in Figure 6 (in addition to the nine
we already plotted), each column ends with a total of 50 hits (Figure 18). The density for any one cell in a
column is its own count value divided by 50, expressed in percent as shown in Figure 19. The math is
very simple: a cell with 24 counts has a 48% density. In practice, instead of batches of 50 waveforms, we
collect a frame of thousands of waveforms before each update to the density bitmap.
Figure 18. Grid showing cell counts after 50 waveforms. For each column, the sum of z-axis values is 50.
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DPX Primer
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