Study On Key Techniques Of AC Impedance Spectroscopy Measurement System Based On DLIA

Impedance is a basic concept widely used in the field of electronics, and is the most important parameter for characterizing electronic materials, components and circuits. More valuable information about frequency characteristic can be obtained by using ac impedance spectroscopy measurement and analysis. Nowadays, precision ac impedance spectroscopy measurement system based on an innovative DSP architecture is the hottest research topic and represents the development trend in the field of impedance measurement. Since it is involved in multiple subjects, high technical threshold, broad application prospects, and its derivative techniques also has good extendibility and reusability, the study on key techniques of digital ac impedance spectroscopy measurement system has very important academic value and practical significance.This paper presents an original digital auto-balancing bridge with wide frequency coverage. A new solution that combines the digital lock-in amplifier with excellent noise suppression ability and the Brent method with super linear convergence characteristic is provided to realize automatic control of the bridge, thus overcoming the deficiencies of the analog auto-balancing bridge. However, as for the normal Brent method, the dichotomy used for locating the boundary can not be used in the complex field. An adaptive quasi-Brent method is therefore proposed to solve this problem. This method unites the definite constringency of the extrapolation and the fast constringency, high accuracy of the iterative method. Compared with the secant method, the response speed and stability are increased significantly.Next, a digital quadrature coherent detection algorithm with the function of vector analysis is put forward by using the coherent detection principle. Compared with the traditional DFT analysis method, this algorithm is simple and is easy to realize. Also, it can be conveniently combined with the digital auto-balancing bridge. On basis of this, a novel dual-phase digital lock-in amplifier is designed to achieve a precise and rapid detection of the unbalanced current and the vector ratio. In practice, the line frequency interference can obviously worsen the signal-to-noise ratio. A multi-feedback type notch filter is designed to solve this problem. Not only can it restrain the line frequency interference, but also its tunable center frequency makes it flexible and adaptable.Then, on basis of analyzing the sampling principle about band limited signals, a mixed sampling method featuring a variable sampling rate is proposed to realize a broad-band, precision and high-efficiency impedance measurement. Compared with the method featuring a fixed sampling rate, the test frequency and efficiency are improved considerably. Meanwhile, the modified digital quadrature coherent detection algorithm reduces CPU usage and storage space requirements, thus avoiding instability due to the discontinuity in output data. Besides, the performance is not degraded. Moreover, a low-jitter sampling clock generator based on DDS is designed to solve the problem that normal DDS clock can not be applied to precision data acquisition because of a relatively amount of jitter.Finally, on basis of the research findings related to key techniques of digital ac impedance measurement, we undertake a major effort to develop a high-performance, low-cost ac impedance spectroscopy measurement system for characterizing PTCR with our own intellectual property rights. The results show that the complete system achieves an up-to-date performance and surpasses other similar products in portability and extendibility. In addition, the dependence of the PTCR on temperature at different frequencies indicates that the impedance measured at ac voltage decreases with increasing frequency. This means that the applications of PTCR are severely restricted to dc and line frequency operation. This conclusion agrees with the said ideas concerning the grain boundary junctions.