The Automatic Frequency Calibration System Design And Realization Based On Direct Frequency Measurement

Frequency standards provide stable running rhythm for electronic system. It is anindispensable important part of the electronic system(radar, communication,navigation, measurement and control, space electronic equipment etc.). It is also thebenchmark of time and frequency measurement. The parameters of frequencystandards will affect the performance of the whole electronic system.With the control technology development of modern electronic, grid,communication, aeronautical and space etc. Requirements for frequency standard ismore and more high. The frequency standards of an electronic system need to haveexcellent stability and high accuracy. As a common frequency standard device, Quartzcrystal oscillator or rubidium atomic clock has a series of advantages, such as low cost,high reliability, short term frequency stability etc. They are widely used in thescenario where the requirements on precision are not harsh. However, since theirfrequency precision is relatively low and they also have the problem of aging driftcharacteristic of frequency marker at different degrees, they can’t be deployed directlyin the time synchronization system with high precision such as updatedcommunication systems, grids. The GPS clock system(or Beidou, GLONASS etc.)Can be synchronously traced back to the Cesium clock in the GPS clock system. Itsoutput frequency has little drifting, and it doesn’t need to be sent to uppermetrological service to get calibrated. Meanwhile it has similar performancecompared to Cesium clock, but it is much cheaper. We can take the GPS(or Beidou,GLONASS etc) clock signal, which has a good long-term stability, as an externalreference time base to calibrate oscillator(or rubidium clock), and lock its frequency,so as to improve the frequency precision and long tern stability of the local oscillator(or rubidium clock). It has mitigated the extent of aging drift characteristic offrequency marker. These frequency accuracy measurement systems combine theadvantage of crystal oscillator(or rubidium clock) and the timing signal provided bythe satellite navigation. So this system has advantages of short-term stability andlong-term stability or high frequency accuracy. We call this system disciplined clocktechnology. The common method of disciplined clock system realization is that wecan compute and control DAC(Digital Analog Converter) module according to 1PPSsignal time interval difference of rubidium clock(or crystal oscillator) and GPStiming receiver, and produce corresponding control voltage to achieve the calibrationof the rubidium clock(or crystal oscillator). This method involves high precision timeinterval counters, frequency dividing or multiplier circuits, and complicated digitalphase-locked loops etc. these all will increase the noise and complexity of frequencyadjustment systems.The frequency correction method designed in this paper has a lot of differencewith the traditional “disciplined clock” frequency calibration system. The difference ismainly reflected in the generated voltage control word, which is from the frequencydifference of crystal oscillator frequency measurement value and its nominal value.The core idea of this method is the direct frequency measurement. GPS(or Beidou,GLONASS etc) clock signal is used as external time base of measuring the frequencyvalue of crystal oscillator. The method simplifies the circuit function, and making thefrequency calibration system has a better anti-noise performance. It has a positivesignificance both on reducing the hardware cost and the frequency calibration systemcomplexity and on increasing the reliability of the frequency calibration system.Through increasing the sampling time of frequency measurement, we can increase themeasuring accuracy of frequency measurement. And reduce the influence of systemnoise by receiver loss of lock and signal jitter. Therefore this system has theadvantages of low cost, small volume, low complexity, high accuracy etc. it has thevalue of further research and development.The author completed the design, implementation, testing and analysis ofautomatic frequency correction(testing) system. And the major work of authordescribed as below.1) using the pulse counting method to measure the frequency of the localoscillator output signal directly. realizing the frequency counter and the frequencydivider based on CPLD.2) According to the frequency counter value calculate and adjust the DAC outputvoltage, so as to control the local crystal oscillator. To achieve the calculation of thecrystal oscillator frequency value(calculation of the frequency counter data), thegeneration voltage Control word and analog voltage(D/A module embedded in ARM),the control of frequency counter and LCD display screen etc based on ARM.3) Control LCD screen display the measured frequency value of quartz crystaloscillator output signal, Control voltage value, the calibration status(whether locked)in real time. To achieve the display function based on LMB162 A.At the end of this paper, The author tests the performance of automatic frequencycalibration system(test) prototype. And statistic and analyse the frequency datacollected. Through the calculation of these data, The frequency accuracy of crystaloscillator increasing of about 1~2 order of magnitude than frequency calibrationbefore. Long term stability was also improved to a certain extent.