The Quality Inspection System Of Digitally Modulated Signals

In digital communication systems, the quality of digital modulation signal has a direct connection with the analysis and reduction of the signal conducted by the receiving device and has a significant impact on system’s reliability. Based on the laboratory’s developing mission of balise transmission control system which is contained in high-speed train control system, this paper describes the quality testing of uplink digitally modulated signals emitted from ground devices. The traditional testing method is using a proprietary instrument to measure different indicators separately by building a testing environment, but it is inefficient. Standing on the platform of Altera Corporation’s FPGA, this paper accomplishes the automatic measurement for the signal envelope, amplitude jitter, MTIE and spectrum of digital modulation signal by using classical algorithms, which improves the testing efficiency greatly. Adopting a lot of pipeline technology as well as ping-pang storage technology, this paper uses software such as Matlab and Modelsim for algorithm simulation and uses Nios Ⅱ soft-core to implement mixed programming with Verilog HDL and C programming languages in the process of implementation. The paper has completed the work as below:1. Research classic amplitude calculating algorithms, including maximum likelihood estimation method, related accumulation method and high order difference method, and make a deep comparison among these three different algorithms in calculation accuracy, computational complexity and calculation time delay. Then select an algorithm which is easy to realize in FPGA, and accomplish the testing for signal envelope and amplitude jitter. Each part of module has a corresponding simulation result and a detailed analysis report.2. Give the definition of MTIE and the commonly used testing method which is based on proprietary instrument. Then put forward two improved calculation methods according to the shortcoming of large amount of computation by using definition to calculate MTIE, one is based on traversal process and the other is based on the observation time window. In FPGA, user-defined logic uses Hilbert transform and table look-up method to calculate the phase, uses the least square method to calculate the time interval error, and uses Nios II soft-core to calculate MTIE with C programming language. The paper not only gives the simulation report for each module, but also introduces how to configure the Nios II soft-core.3. Make a comparison and an analysis of classical and modern spectral estimation and analysis methods, and use AR model parameter estimation program to achieve the spectral measurement which includes parameter estimation part and spectrum estimation part for digital modulation signal. The parameter estimation part adopts Burg algorithm which is based on linear prediction to calculate the reflection coefficient, the forward prediction error, the backward prediction error, the parameter model and the error power by using pipeline technology, whereas the spectrum estimation part is accomplished by using look-up table method.The paper not only describes the derivation of each algorithm in detail, but also gives the simulation results.This paper accomplishes the quality inspection of digital modulation signal in balise transmission system, which has realistic significance in practical applications.