| Under the development of international satellite navigation systems, it is meaningful to make research on signal simulator of GALILEO. This paper is based on the development of GALILEO signal simulator and focuses on the key techniques of it.A detail introduction of the structure and the components of the system, as well as the comparison with GPS is shown in this paper at first. After that the thesis presents the frequency allocation, signal structure and modulation of GALILEO system, which offer the evidence of the simulator design.To explain the key technology, the spread spectrum theory, the DS-SS characteristic and the GOLD sequence are given at first, then the article makes a research on the frequency synthesizer and compares the differences between PLL and DDS, which offer the theoretical basis of the simulator scheme.According the above work, the general plan of the GALILEO signal simulator and the structure are derived. The pseudo sequence generator is designed based on FPGA in accord with the GALILEO characteristic and its demand on the performance of random code. After the understanding of the features of PLL and DDS, the carrier generator module based on DDS is designed. The carrier modulator is carried out after the study of BPSK. A developed method of twice up-mixing module is brought forward after a thoroughly study advantages and disadvantages of three kinds of DDS+PLL which are usually used in practice.The detail process of realizing the signal simulator is displayed using actual circuit design and simulation when the plan has completed. The pseudo code generator is done using FLEX10K130E after a thoroughly study of the Altera FPGA chips and its developing environment Quartus II Also AT89C51 based carrier frequency synthesizer, carrier modulator and the mixing circuit are realized after the study of AD9854,wide band dual inputs operational amplifier OPA678, ADF4113. Finally the paper discuss the idea of the simulation of Doppler frequency with the simulator. |
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