Tektronix SPECMON3 Manual page 73

General Signal Viewing
Figure 30. The analyzer triggered when the density in the DPX measurement box exceeded the threshold set by
Trigger On This. You can see in the Spectrogram and Frequency-vs-Time displays that the signal event which caused
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 firing. For the
common configuration 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 fire 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 fire 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
SPECMON3 & SPECMON6 Printable Help
DPX Primer
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