Tektronix RSA6100A Series, Help Manual

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General Signal Viewing > DPX Spectrum DPX Primer
the trigger was a quick frequency hop. The Time Overview shows that the signal amplitude never changed, so a
power level trigger would not have worked.
DPX Density Trigger Timing
The time resolution for DPX density measurements is the frame length, around 50 ms. A basic
implementation of the DPX Density trigger concept is also frame-based, so a trigger event that occurs
anywhere within a frame will not be recognized until the end of the frame. Therefore, the worst case
trigger uncertainty is 50 ms.
DPX Density trigger doesn’t always have to wait until the end of a frame before fi ring. For the
common con fi guration of triggering when the measured density is higher than the threshold, the density
measurement in the trigger can be computed many times within each frame and it can fi re the trigger
as soon as the threshold is exceeded.
Consider the case where the threshold is zero. As soon as a single waveform causes a hit within the
measurement box, we know that the density is greater than zero. It takes a little longer to test for a 5 or
10% density, and even more time for thresholds at or near 100%.
The DPX Density trigger can also be set to fi re when the measured density is below the threshold value.
This is useful when you suspect that your signal is missing some of the time. For a signal that is supposed
to be CW, you can set the trigger controls to acquire when the density measurement of the signal peak
drops below 100%. When using the “lower than” form of the DPX Density trigger, the time resolution is
one frame because of the following logic: We can’t be sure the actual density is less than, say, 15% until at
least 85% of the full test time has elapsed. In order to keep things simple and fast in the trigger module, the
RSA just waits until the end of each 50 ms frame to do the “lower than” comparisons.
Persistence and DPX Density Trigger
The smoothing effect of persistence on density measurements can help in determining a good threshold
value. With persistence turned off, an infrequent signal’s density reading jumps between higher and lower
values as it turns on and off, and it can be hard to read these
fl ashing numbers. By turning persistence
on, you instruct the instrument to average the density over a longer time period. This density result is
somewhere between the ON and OFF density values - the very de
fi nition of a good trigger threshold.
Unlike the DPX Density measurement, the DPX Density trigger is not affected in any way by persistence.
Density calculations in the trigger system are made with hit count data received from each individual DPX
frame, before any persistence is applied. Even when the density measurement reading in the display
is averaged over many frames due to persistence, the trigger is computing density for each frame and
comparing these quick snapshots against the threshold setting.
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RSA6100A Series Real-Time Spectrum Analyzers Help