The Develop Of High Performance Microwave Frequency Synthesizer Based On DDS+PLL

The continuous development of communication system has put forward a higher and higher demand on the stability and accuracy of the frequency source. As a well developed frequency synthesis technology, PLL (Phase Locked Loop) is good at spurious behavior and wide frequency bandwidth, but it has weakness on frequency switch speed and frequency resolution. The DDS (Direct Digital Synthesis) technology has attracted popular attention for its excellent performance on frequency switch speed, frequency resolution and phase noise. However, due to its digital structure limit, it just can work on narrow bandwidth and its spurious level is a serious problem.This paper integrates the DDS and PLL technologies. A frequency synthesizer applied to GSM 1800MHz,1900MHz is finished with the hybrid frequency synthesis method that DDS directly drives PLL. The synthesizer works well from 1700MHz to 2000MHz which can output arbitrary frequency with 200kHz frequency step. The spurious level and phase noise performance meet the requirement of actual Project.At first, this paper describes the basic structure, working principle, sources of spurious signals and characteristics of output spectrum of DDS. Then presents the basic structure, working principle, and analyzes the loop linearization, stability and noise characteristics of phase locked loop(PLL). Some hybrid frequency synthesis methods for DDS+PLL which used usually are demonstrated and selection in the following section. According to the system requirement specifications, the excellent performance of the DDS and PLL chips are chose to control frequnency of the system plus 8051 MCU. A simulation platform is established which make use of ADS and ADIsimPLL simulation software for the various modules of the system simulation analysis and parametric optimization. The entire system PCB(printed circuit boards) is mapped by Protel99se. Finally the hardware of the entire system are completed and debugged. The spectrum analyzer analyzes the output frequency spectrum, and test results show that the system achieves the required specification.