High precision time interval measurement in pulsed laser ranging

The high accuracy time interval measurement is a key technique to a pulsed laser ranging.In this paper,a new high precision time interval measurement method combining with a pulse counting method,a multi-phase sampling and a delay method is proposed to use in the multi-level time interval measurement in a Field Programming Gate Array(FPGA).The pluse counting method is used to realize the coarse counter to guarantee a large dynamic range,and a Phase Locked Loop(PLL) in the FPGA is used to generate the N clock pulses with the same frequency and uniform distribution phase as counting clocks.Based on the equal precision frequency measuring,the resolution of the time conversion is improved to Tclk/N.Moreover,a latch Flip Flops is used to form a delay chain to measure the fine time interval.Experiments show that the proposed method has solved the problem of the traditional multi-phase sampling approach,to which phase shift resolution is decreased with the frequency increasing,so that the fine resolution can be obtained by limiting delay lines and no increase of counting clocks.The test results show that the maximum measuring time of the module is 163.8 μs,the conversion time is short and the standard error of the measurement is less than 71 ps when repeated measurements are carried out,which can satisfy the requirements of higher precision systems.