The Design Of High-precision Measurement System For High-impedance & Low-frequency Crystal Oscillator

With the rapid development of the Internet of Things technology,various embedded devices have emerged one after another,in the meanwhile,being clock sources,crystal oscillators have been applied in many aspects.Among them,low-frequency tuning fork crystals are used to provide RTC clocks,and the annual consumption is up to tens of billions.The measurement of the electrical parameters of crystals is a key step in the production of crystals.Compared to high-frequency crystals,the test for low-frequency crystals is much more difficult due to the high resonance impedance of low-frequency crystals.There is no measuring equipment in China that can meet the high-precision online calibration requirements of low-frequency crystals.Aiming at this problem and combining the production line requirements of a domestic crystal production company,this paper designs a crystal measurement system for high impedance characteristics to achieve high-precision measurement of low-frequency crystals.This paper comprehensively analyzes the equivalent structure,oscillation principle and international measurement standards of crystal oscillators,determines the key indicators in crystal oscillator measurement technology,and proposes a measurement scheme based on the frequency-phase transmission method in S-parameter of two-port network.For the problem of high impedance of low frequency crystal and high insertion loss in standard impedance system,a low frequency crystal matching network with attenuation as low as-30 d B was optimized to reduce signal loss,and a configurable signal gain system was built to reduce crystal Excitation power dependence.As for the system response time and measurement accuracy index requirements,a high-precision frequency sweep signal source with a direct digital frequency synthesis core is designed,and the overall measurement speed is increased by compromising the equalized frequency conversion time and the resolution of the frequency sweep signal;At the same time,the principle of logarithmic demodulation and phase reduction filter phase detection is introduced to construct a gain-phase detector to replace the traditional potentiometer detection structure,which effectively enhances the resolution accuracy of the data acquisition terminal.In the end,this article uses Altium Designer to design the hardware circuit of the system,build a software platform based on the MFC,complete the overall implementation of the system,and measure a batch of crystal samples to compare with their standard parameters.The experimental results show that the measurement accuracy of the resonance frequency of this system is as high as ± 1ppm,the error of the resonance resistance,equivalent inductance and dynamic capacitance is less than 5%,the quality factor accuracy is 5%,the DLD2 parameter deviation is less than 20%.The average single measurement time is less than 1s.To sum up,the low-frequency crystal oscillator measurement system can meet the measurement index requirements of various parameters in the existing crystal oscillator production,and it achieves low-power,high-speed,high-precision low-frequency crystal oscillator measurement.