Trigger Holdoff

Controls: Trigger Dialog >> Holdoff
Macro: TrigHold

Trigger Holdoff is the minimum interval between triggers. It is useful for signals like repeating tone bursts, where you want to trigger on the start of each burst, not on some arbitrary cycle of the waveform within the burst. Without Holdoff, any one of the cycles in a burst might be an acceptable trigger.

To use Holdoff, the Trigger Mode should be set to Normal unless you are sure that the burst repeat period is less than the duration of the unexpanded waveform display (21 msec for the default 48000 Hz sample rate.) Then you may use Auto or Auto Level.

The display will not be stable until Holdoff is properly set, but you should be able to see signal bursts rolling past. Set Trigger Level to a value about half that of the visible peaks. The display still won't be stable, but at least it should always start at the same vertical extent of the waveform.

The Holdoff value must be shorter than the overall burst repeat cycle by at least one cycle of the carrier frequency (the waveform that appears during a burst). It must also be at least as long as one burst (less one carrier cycle).

If you know the burst duration, set Holdoff to that value. (Note that for Holdoff purposes, the relevant duration is actually the time the burst exceeds the Trigger Level, not the overall burst duration. This is usually only an issue for bursts with slow rise and/or fall times.)

If the burst duration is variable, set Holdoff to the maximum value. If the repeat cycle is variable, set Holdoff less than the lowest repeat cycle.

For example, suppose you have a 500 Hz tone burst that varies between 10 and 15 msec, and the overall repeat cycle varies between 40 and 60 msec. One cycle of the 500 Hz carrier is 1/500 = 2 msec. Holdoff must be greater than (15 - 2) and less than (40 - 2) msec.

If you are not sure of the durations involved (which may be the case if you are not controlling them yourself, and you can't get a stable trigger to view them until Holdoff is properly set), then take a guess. You should deliberately make your guess a little too high or too low. If you start with a low value, slowly increase it until you get a stable display. If you start with a high value, slowly decrease it.

If the burst durations or repeat cycle times are variable, the display may seem to be stable for a while, then roll when the duration becomes too long or the repeat cycle too short. If so, keep changing Holdoff in the same direction until the display remains stable. If you go too far and it becomes unstable again, back off to a medium value.

The small button to the left of the Delay control above Holdoff toggles between 'Smpls' (samples) and 'sec' (seconds). You can enter or view in whichever units you choose, and switch back and forth at any time.

When Trigger is off (free-run), Holdoff may be used to control the overlap or gap between trace updates, since it is the minimum time (or number of samples) between updates. If you are viewing unPaused file data, however, the Holdoff value is ignored when Trigger is off. Instead, the display advances by the Read Step Size on every update.

Macro Notes:

The Smpls/sec state determines the units used when setting Trigger Holdoff, as well as Delay. SmplSec=sec sets the units to seconds, as does SmplSec=1. SmplSec=Smpl or SmplSec=0 sets Samples, and SmplSec=x toggles between the two.

TrigHold=100 sets Trigger Holdoff to 100 samples if the units are Samples, else 100 seconds.

Alternatively, TrigHold=>1 increments Holdoff by 1 sample and TrigHold=>-1 decrements by 1 sample. The step size is the same for seconds mode, but the display shows the equivalent seconds.

See also Trigger Toggle - Introduction, Trigger Control Dialog