Method and circuit for triggering an electronic instrument only once

Abstract

A method and circuit for triggering an electronic instrument only once during a period of an input signal having multiple triggering events during that period. A signal applied to an electronic instrument, such as an oscilloscope, is acquired. Qualified triggering events occurring on the acquired signal are identified. A selected one of the qualified triggering events during the period of the applied signal causes a trigger signal output to occur, while other qualified triggering events are ignored. In the preferred embodiment a triggering event is qualified by a first comparator that compares the input signal to a first reference level and produces a predetermined logic level output if the input signal bears a predetermined relationship to the reference level. When a qualified triggering event occurs, a buffer amplifier applies the predetermined logic level to charge a capacitor, which thereafter stores some energy from the input signal and applies it to a comparator for a predetermined period of time. The comparator compares the voltage on the capacitor to a second reference level and produces a pulse if the capacitor voltage has a predetermined relationship to the second reference level. The leading edge of the pulse comprises a trigger output signal. Subsequent triggering events which occur before a predetermined period of time are unable to produce another trigger signal. Five alternative embodiments employing the same general principle as the preferred embodiment are disclosed.