Study On A High Precision Digital-Analog Mixed Temperature Compensated Crystal Oscillator

In recent years, high precision crystal products in the electronic market is beingapplied more and more widely with irreplaceable advantages because of rapid growth inelectronic market demand, especially the temperature compensated crystal oscillatorwhich has the advantage of high compensation precision, diversification incompensation ways and low cost is widely used in the electronic market. As the systemclock source, the crystal oscillator’s frequency shift along with the change oftemperature. The unstable clock source will results in the abnormality of the system.After temperature compensated, the crystal oscillator’s frequency is relatively stable andits application is more extensive.Firstly, this paper studies the temperature compensation method of crystaloscillator, then the temperature compensation method combined analog compensationand digital compensation based on the standard CMOS process is proposed. Thismethod can not only overcome the shortcomings of low precision of analogcompensation, but also reduce the complexity and cost of the process. Analogcompensation network in this paper mainly consists of cubic function generator andlinear curves family. One can select the linear curve according to the AT cut crystal’sangle which can avoids the analog compensation results changing widely. Digitalcompensation network is mainly composed of ADC, non-volatile memory (NVRAM)and DAC. The NVRAM implemented in standard CMOS process, which can reduce thesystem process complexity and avoid the limitation of EEPROM process. This paperproposes a kind of hybrid compensation algorithm, which is completed based on varioustest modes and each test mode is completed based on test instructions, these testinstructions constitutes a test instruction set which can ensure the test successful.In this paper, the system is verified by simulation. Experiment results show thatthis system has realized the high precision temperature compensation in temperaturerange-40℃~85℃, and has the advantage on the high level of integration, low powerconsumption and low process.