Research On Low Power Integrated High-precision Timekeeping Technology For Navigation

With the wide application of satellite navigation and positioning system in the world, fast acquisition and tracking, high precision positioning, high availability, low power and miniature of the satellite navigation receiver is required, especially in the scenarios of urban canyon, culverts, overpasses, which will shield satellite navigation signals. The fundamental satellite navigation receiver operation is computation of distance and position through measurement of the time it took for special radio signals to travel from a satellite to a receiver. The time keeping in receiver played a crucial part in measurement of the time is an important influent factor to computation of position. So the research on low power integrated high-precision timekeeping technology for navigation will have important engineering value. In that background, this paper pays attention on the following research work:(1) The AT-cut crystal widely used in portable navigation equipment suffers from lack of self-temperature-sensing. To overcome that limitation, this paper proposes a novel dual-crystal self-temperature-sensing method. By employing two AT-cut crystals with similar frequency but different shearing cuts, which have frequency-temperature characteristic, the proposal method uses the beat frequency to represent the crystal temperature. And it can avoid inaccuracies due to a difference in crystal and thermistor effective thermal time constant. The analysis and experiment results showed its temperature resolution is better than 0.4390℃/Hz, which is the same accuracy as the expensive and complicated SC-cut crystal dual-harmonic self-temperature-sensing method.(2) A new modeling method of composite AT-cut crystal frequency-temperature characteristics is suggested in this paper, which can improve the frequency-temperature stability when compared to conventional AT-cut crystal temperature compensation. In this method, a frequency-dynamic-temperature characteristic model is established after thorough analysis the origin of frequency shifting under dynamic temperature variations. And an approach is suggested, according to which the AT-cut crystal frequency shifting by real thermal hysteresis is formed as a function of temperature variation history. The analysis and experiment results confirmed that the application of the composite modeling method and dual-crystal self-temperature-sensing could reducing frequency-temperature model error from the conventional 1×10-6 to 5×10-9,which is at the same accuracy of SC-cut frequency-temperature model.(3) For today’s portable navigation equipment, a crystal oscillator above megahertz with a current of milliamperes is unsuitable, which is usually created based on the classical oscillation model of small-signal circuit impedance. A new design method for megahertz crystal oscillator based on an oscillation model of circuit positive feedback is presented. It has less error in estimate of stable oscillation current and additional ability to quantify the condition for initial oscillation, which means higher accuracy oscillation control and lower power consumption. Experiment results showed that the power consumption of 6MHz and 12 MHz quartz oscillators based on the new design method is below 1μA, and a three orders of magnitude fall compared to the previous research work.(4) Generally, the conventional timekeeping based on TCXO can not achieve low power consumption. This paper supplies a digital timekeeping technology employing software to temperature time-base compensation. By this technology, the time is compensated directly according to temperature, and the additional low-power operation state is adopted, which can simplify system and reduce power consumption to 8.5μA. The compensation software based upon high-precision frequency-temperature model can avoid the conventional compensation accuracy constraint by the pullability of crystal, whose compensation accuracy is better than 5×10-9,and as same as the SC-cut crystal temperature compensated timekeeping.(5) The present TCXO test systems are of low test efficiency and restricted to record frequency characteristic under static or quasi-static temperature variations. A parallel frequency measurement method is described in this paper and a parallel TCXO test system using this method was developed. This method is based on directly frequency measurement and able to measure frequencies parallel. And at least 100 TCXOs can be measured simultaneously in the proposed test system under static or dynamic temperature variations, and its test capacity can fully meet the efficiency requirement of the design and production of state-of-the-art timekeeping chips.The research has been applied in the developing of satellite navigation receivers in China’s BDS and solved the lack of high-precision, low power, low cost and miniaturized timekeeping chips. And it has theoretical significance and engineering value as well.