Abstract: A signal responsive burst period timer and counter is provided for laser Doppler velocimetry, and the like, by bandpass filtering a signal burst and testing the filtered signal for exceeding a first level (V.sub.O -V.sub.L) and a second level (V.sub.O +V.sub.L), and subsequently crossing a reference level (V.sub.O) from the second level, and in that order, to qualify each cycle of the signal burst for timing and counting. A timing system checks the reference crossing of every qualified cycle, in accordance with the level crossing logic criteria described above, to determine that it occurs within a prescribed time interval as measured from the previous qualified reference crossing. When the sequence ends, either because a predetermined number of qualified cycles have been received or because the signal fails to meet one of the amplitude or time criteria described above, the burst period counter stops, the data is read out and the counter resets itself to process the next burst.