User manual

Programmer’s Guide Page 47 of 64
3.12. Trigger Delay and Horizontal Waveform Position
When using a digitizer the user has 3 instrument setup variables with which to position the acquired waveform in
time:
• sampInterval: the sampling interval (inverse of the sampling frequency)
• nbrSamples: the number of samples to acquire
• delayTime: the nominal trigger delay
By convention, the nominal trigger delay is taken relative to the beginning of the trace, i.e. relative to the left edge of
a real or virtual display grid. It can be interpreted as the time from the trigger to the start of waveform recording. If
this number is positive, recording starts after the trigger (post-trigger acquisition). If it is negative, recording starts
before the trigger (pre-trigger acquisition). In reality, the acquisition always runs before any trigger occurs, and
delayTime controls the time between the trigger and the stopping of the acquisition:
delayTime
Time until Acquisition Stop
Comments
- sampInterval
* nbrSamples
0 Trigger point is at the right edge of grid, i.e. at the end of
the nominal waveform (100 % pre-trigger)
< 0 sampInterval * nbrSamples
+ delayTime
Trigger point is at the desired point within the grid
0 sampInterval * nbrSamples Trigger point is at the left edge of grid, i.e. at the
beginning of the nominal waveform (0 % pre-trigger)
> 0 sampInterval * nbrSamples
+ delayTime
Trigger point is to the left of the grid, i.e. before the
beginning of the nominal waveform (post-trigger)
Note that delayTime is not allowed to become more negative than - sampInterval * nbrSamples, because it is
impossible to stop the acquisition before the trigger occurs.
3.13. Horizontal Parameters in Acquired Waveforms
Triggers usually occur asynchronously with respect to the sampling clock. Thus, between similar events, the time
from the trigger to the next sampling clock varies randomly in the range [0 … sampInterval].
The true time reference for any waveform is the trigger point, not the sampling times, because the trigger is attached
to a given feature of the waveform (e.g. a transition at a predetermined level). For highly stable displays, it is
important to know the time between the trigger and the next sampling clock to within a fraction of the sampling
interval, and to place the displayed data points in such a way that the trigger point stays at a constant position. This is
particularly important for persistence displays or highly zoomed random-interleaved displays, as generated from
overlaid segments, where a single waveform (or waveform segment) contributes only a few data points to the
display.
Trigger
delayTime
sampInterval
nbrSamples