Clock-induced bias errors in laser Doppler counter processors

Laser Doppler anemometer measurements using counter processors and requiring large shift frequencies are sensitive to small errors introduced by limited digital resolution as well as the method of clock pulse counting. Tests with at least one commercially available processor have shown the existence of a bias error induced by the clock pulse counter. This error has a significant effect on results for a low velocity flow component when a large shift frequency must be used for all components in order to avoid fringe bias. A correction method is proposed.