The Design Of L-Band Frequency Synthesizer Based On The DDS & PLL Hybrid Method

Frequency synthesizer is an important component in the modern electronic systems. Ithas popular application in the field such as radar, communication, measurementinstruments and electronic navigation, etc. As a well-developed frequency synthesistechnology, PLL (Phase Locked Loop) is good at spurious behavior, but it has weaknesson frequency switch speed and frequency resolution. The DDS (Direct Digital Synthesis)technology has attracted popular attention for its excellent performance on frequencyswitch speed, frequency resolution and phase noise. However, due to its digital structurelimit, it just can work on a narrow bandwidth and its spurious level is not so good.This paper takes the DDS and PLL technologies together.A L-band frequencysynthesizer is finished with the hybrid frequency synthesis method that DDS directlydrives PLL. After a detailed debugging, the synthesizer works well from 1400MHz to1600MHz, it can output arbitrary frequency with 1kHz frequency step. The spurious leveland phase noise performance meet the requirement of actual project.The main devotement of this dissertation is as follows:1. After a detailed analysis on the basic principle of DDS and PLL technologies, thehybrid frequency synthesis method DDS directly drives PLL is adopted toovercome the contradiction between frequency resolution, lock time, frequencyagility speed and phase noise.2. Complete the control circuit with a high-speed FPGA chip. This design canweaken the impact of control circuit on the frequency hopping speed.3. Frequency sweep function is realized, and have 5MHz, 1MHz and 0.1MHz threeselectable sweep mode.4. Finish the PLL circuit with a 4th loop, several important parameters of the PLLare derived from the basic theories. After a detailed debugging, the spurioussignal generated by the DDS circuits is successfully eliminated.5. A conclusion of the DDS direct drive PLL hybrid frequency synthesis method is presented, and several useful methods to improve the performance of thesynthesizer are discussed.