A circuit is described which synchronizes the timebase of a transient waveform digitizer to a randomly occurring trigger signal synchronous with the experiment information which is to be digitized. The method described overcomes the triggering ambiguity of the recorder and, therefore, uncertainties in time origin of the recorded signals. Pretrigger information is still preserved accurately except for a±1 sample period uncertainty prior to the trigger event. The design is based around a 200‐MHz sample rate digitizer and uses a clock generator, which can be synchronized within one period to an external event. The same technique is applicable to slower clock rates and up to ∼500 MHz.