Reliable trigger detection uses two thresholds (hysteresis) so one rising edge fires only once

A trigger input that simply fires whenever the voltage crosses a single fixed level will misfire when the source is a steep, bandlimited digital edge: the ringing overshoot (Gibbs phenomenon) around a fast rise crosses that level many times, producing a burst of false triggers — as happens if you use a virtual VCO square wave as a trigger source. The fix is hysteresis: require the voltage to rise above a higher threshold (around 1–2 V) to fire, and to fall back below a lower threshold (around 0.1 V) before it can fire again. Because those two thresholds differ, a single noisy rising edge can only fire once. This is the general principle behind clean edge detection for clock, trigger and reset inputs, whether in software or hardware.

Examples

A 1 kHz square wave fed to a naive ‘fire if V > 0.5’ input triggers thousands of times per second from ringing. With a high threshold near 1 V and a low threshold near 0.1 V, it fires exactly once per cycle on the true rising edge.

Assessment

Explain why a single-threshold trigger input misfires on a bandlimited square wave, and how using two different thresholds (a high fire level and a lower re-arm level) guarantees one trigger per rising edge.