Cycle Counting Function of TSI 1990A and 1980 LDV Processors as a Means ofParticle Transit Time Measurement

摘要

The main objectives are to study the dependence of the number of counted cycleson signal level, and to check that the cycle counting process is independent of the Doppler frequency. Tests of TSI 1990A and 1980 counters are performed with different kinds of stimulated signals generated by function generators. Measurements show that the number of counted cycles depends on the ratio of signal to threshold detector levels in a predictable way. The cycle counting process is not frequency dependent. Indirect accuracy limitations will arise from the dispersion introduced by the effect of signal level. Major difficulties were encountered because the signals used for test purposes were different from actual laser Doppler velocimetry (LVD) signals, causing wrong cycle counts: occurrence of end of burst failure due to improper signal shape, and interference of the dead time of the processor with cycle counting. Other difficulties are related with signal shape imperfections. In actual measurements errors of the same kind are likely to occur owing to the lack of validation of cycle counts. Requirements for a suitable cycle counting processor, and the conditions of proper signal generation for testing Doppler processors are defined.