High Precision Absolute Gravity Meter Based on Time-to-Digital Conversion in FPGA

This paper proposes a novel method to design a high precision portable gravity acceleration meter on the Field Programmable Gate Array（FPGA） platform. Two technologies are used in FPGA to improve the time measurement accuracy to 54 ps and the measurement accuracy of g to the level of 1 μGal（1 Gal=1 cm/s;）. The one is the proposed clock rising edge counting method for measuring the coarse time, in which the measured coarse time resolution can reach 5 ns by setting the clock frequency up to 200 MHz. The other is the realization of the time-to-digital conversion（TDC） circuits in FPGA which can further improve the time measurement accuracy to 54 ps. In the TDC circuit, we analyze the influence of the frequency stability of crystal oscillator and the distance of infrared tube on the measurement accuracy of g. The increase of frequency stability(10;to 10;) can greatly improve the measurement accuracy of g（5.814 3 to 2.079 9 μGal）, but further increase of frequency stability(10;to 10;) has little effect on improving the accuracy of g. Through analysis and simulation, it is found that the setting of distance between laser pair also has a great influence on the accuracy of g. Only when the distance is set to the optimal, we can make the accuracy of g the highest. In this paper, the method of obtaining the best distance and the corresponding accuracy of g are given. In addition, the proposed system uses Nios II soft core processor to implement all the controlling, calculation and display functions, which improves system integration, reduces system cost and meets the needs of field geological survey and gravity prospecting.