The Research And Design Of Preselected Filter And Local Oscillator In Ultra-short Wave Receiver

In recent years, communication technology has been an unprecedented development.The industry can not do without signal. Because of its excellent performance, ultra short wave receiver has become the darling of the field. In order to achieve an efficient and accurate signal, radio frequency engineers has been to improve the structure of the receiver, and no matter what kind of structure, the design of RF front-end of receiver is the key of the entire system, it is important to the whole system of the receiver sensitivity, dynamic range, selectivity and anti-interference performance index function. Because of this, it is necessary for us to conduct the thorough research to the receiver RF front-end, more dash forward show actually use value.The subject completed the design and implementation of two key modules of RF Front-end in ultra-short wave receiver. First,this paper provides a design scheme of filter which has good selectivity and sensitivity. According to the norms of the project, the structure of the filter is two inductively coupled double tuned circuit cascade form,and according to the actual test data, the low noise amplifier is inserted between the two stages, in order to compensate the error of the amplitude frequency of the signal. Due to the resonant frequency of the filter in the 30MHz~88MHz index can be changed, so by a tuning voltage to control the switch diode BA277 pass or not, and then change the resonant capacitor size, this capacitor and resonant inductor constitute the LC resonant circuit in order to realize the fast tuning of the signal.If an appropriate value of capacitor array, the resonant frequency and the binary code shows good linear, so that you can using external digital signals to control it, the resonant frequency of the filter can be adjusted.Secondly, the article provides a high-precision, low phase noise local oscillator in order to meet two times frequency change of uperheterodyne receiver. The basic principle of PLL frequency synthesis and direct digital frequency synthesis are analyzed,then the characteristics of the output signal is also analyzed, especially the influence of phase noise and spurious to the receiver. Through the comprehensive judgment of two kinds of synthetic technology of frequency synthesis, the design of hardware frame is accomplished by PLL+DDS. Using the DDS high resolution, low phase noise and PLL frequency and bandwidth advantages, the reference frequency from the ocox is converted into 800 MHz signal for DDS1, the frequency range of the output signal is between 180MHz-238 MHz,then complete the mixing with the receiver RF signal. Because the receiver uses heterodyne two mixing structure, thus, the signal output from PLL generates a 108.5MHz signal through the DDS2 module, and then complets two mixing with the first intermediate frequency of the receiver. Because the signal exists loss and high frequency clutter after the DDS output, so the broadband amplifier module and filter module are added in the circuit inorder to complete the final output of the signal.The test results show that,in the range of allowable error, the two modules can both meet the design requirements, it fully verify the feasibility of the design.