Design And Implementation Of Digital Frequency Measurement System

As one of the seven international standard physical quantities, time is of the highest measurement accuracy currently achievable. The advancement in time and frequency measurement has a positive role in improving the overall level of science and technology.Timing, telecommunication, navigation and positioning are inseparable from time and frequency measurement. With the rapid growth of economy, technology as well as national defense, the accuracy of frequency standards has become increasingly demanded. Enhancing the research level of frequency standards, improving the manufacturing level of frequency sources, getting rid of the situation that most testing equipments and instruments depend on import has become an urgent and key problem to be solved.Based on traditional frequency measurement methods, a new digital measurement system is designed which is the combination of digital down conversion and phase alignment algorithm. The method is built on two principles: ML phase estimation algorithm and signal subsampling. Utilizing the characteristic that the low-frequency analog signal after undersampling remains equivalent initial phase information of high-frequency signal before sampling. Reference signal and signal under test are simultaneously sampled by dual-channel ADC. The digital data which carries phase information of both signals is sent to SDRAM cache. Then it could be read by FPGA and be transmitted by USB controlling chip. As the form of data stream, the data is delivered to PC. The latter analyses the data acquired using ML estimation algorithm and calculates the corresponding frequency difference information.With the sampling interval in the range of 10ms, the measurement accuracy is better than 10-10, noise floor is in the order of 10-13.In this article,hardware and software design of digital frequency measurement system is described in detail. First of all, the feasibility of design is proven in theory and all factors which would affect the system are analyzed comprehensively. Secondly, through the repetitive tests of the system, the specified performance indexes are calibrated. Besides, the system has characters of high-stability, simple-structure and high-precision. Testing indicators have reached the competitive domestic level. More importantly, it provide a digital frequency measurement thinking and lay the foundation for future improvement based on traditional frequency measurement means.