pretrigger

A pretrigger is a buffer of data or time captured before a trigger event actually fires. In oscilloscopes, data acquisition systems, and waveform recorders, the pretrigger function stores samples leading up to the moment that the instrument detects your signal of interest. This solves a fundamental problem: by the time your trigger condition occurs, the interesting behavior has already begun. Without pretrigger capture, you miss the rising edge, the leading transient, or whatever precedes the event you want to measure.

Pretrigger depth is expressed as a percentage of total memory or a fixed sample count. An oscilloscope set to 20 percent pretrigger and 80 percent posttrigger will capture 20 percent of its waveform before the trigger point and 80 percent after. Some instruments allow pretrigger to consume 90 percent or more of the capture buffer, useful when the signal you need to see arrives well before the trigger edge. The pretrigger window is held in RAM and continuously refreshed; only when the trigger fires is the data locked in memory and made available for analysis or export.

Practical constraints and trade-offs

Pretrigger capture consumes memory bandwidth and storage. A digitizer sampling at 1 gigasample per second will burn through buffer space quickly if pretrigger is set deep. Real-time oscilloscopes with finite memory cannot capture indefinitely before triggering, so users must choose a pretrigger depth that balances the need to see pre-event behavior against the sample rate and record length they require. Repetitive or single-shot captures have different pretrigger strategies: repetitive captures can afford shorter pretrigger because the same event occurs many times, but single-shot captures often demand maximum pretrigger to avoid missing one-time transients.

The term arises naturally from triggering logic: pretrigger is the phase that runs before the trigger activates. In test and measurement, pretrigger is essential for characterizing power-on behavior, fault sequences, ringing, and overshoot. Embedded logic analyzers and protocol decoders also rely on pretrigger to show the bus state before a detected packet or error condition. Without it, troubleshooting electrical events becomes a game of guesswork about what preceded the measured fault.