The Research And Implementation Of High-precision LO Signal Source On Short-wave Receiver

Short-wave is used for wireless communication which the wavelength is from 10m-100m. Regardless of being in the early time of wireless communication invention, or in modern time of communication rapid development, Short-wave wireless communication is one extremely important mean of communication, and is being in unceasing development and consummation. The paper does the research on the connected circuit of the short-wave receiver front-end, and designs the local oscillator of the circuit in short-wave front-end using Direct Digital Synthesis Technology.Frequency synthesizer is one of the most important parts used in modern electronic systems. With the development of modern communication techniques, frequency synthesizer is required to have higher performance such as wide band, fast frequency switching speed, low spurious level, pure output spectrum, high resolution, etc. The important performance of frequency synthesis is the stability of frequency and phase noise. The performance of system will decrease if the factors of clock source can not reach our ask demand. After comparing kinds of technique of frequency synthesizer, we choose the direct digital frequency synthesis chip to produce high-performance and functionally-integrated LO signal source with low phase noise and spurious, and gives the tracking filter circuit design. The scheme is innovative and original not only in the technology but also in some module circuit.This paper first reviewes the development history of receiver, compares with the characteristic of superheterodyne receiver and zero-IF receiver. introduces the concept of frequency synthesis, it has analyzed the principle and the basic structure of DDS. Secondly, description the impact of LO signal on Receiver, it also has carried on the theoretical analysis to the ideal DDS frequency spectrum characteristic and spurious suppression characteristic, the spurious original from phase truncation error,amplitude quantization error and DAC conversion error is analyzed, also an analysis is made on spectrum of LO signal. Then according to the target system, a signal generator with the brief structure and excellent performance is designed with adopting DDS technology, which is based on the AD company's DDS chip—AD9953. In order to obtain high precision, the frequency bandwidth of LO signal, the design of a tracking filter to reduce the clutter and ensure the purity of the output signal spectrum. The system structure, the design of software and hardware and the way to realize the circuitry of the LO signal generator is analyzed in detail. At last, the corresponding PCB has been made and debugged, it has given the picture of the hardware circuit and the test result, and some analysis is made for it.