The Design Of Filter In PLL+DDS Hybrid Frequency Synthesizer

This paper mainly studies and designs the bandpass filter in PLL+DDS hybrid frequency synthesizer with out-of-loop mixing.Firstly,the basic principle,noise characteristics and spectrum performance of each module in PLL+DDS hybrid frequency synthesizer with out-of-loop mixing are described.The bandpass filter is an important part of the PLL+DDS out-of-loop mixing mode,which directly affects the spur characteristics,phase noise characteristics and output frequency range after mixing output.Therefore,this paper focuses on the research and design of bandpass filter in PLL+DDS hybrid frequency synthesizer with out-of-loop mixing,and carries on the test and analysis.In order to reasonably design the bandpass filter that meets the requirements of the system,the basic principles of each module of PLL+DDS hybrid frequency synthesizer with out-of-loop mixing are analyzed and studied in this paper.According to the design goal,each component module in the system is selected,including ADF4351 phase-locked loop,AD9910 direct digital frequency synthesizer,ADGM1304 single-pole four-throw MEMS switch and microstrip bandpass filter.Secondly,according to the frequency spectrum of mixing output,it is concluded that the bandpass filter in this structure has a higher demand for the stopband rejection,therefore,a narrow-band bandpass filter is designed,but in order to widen its bandwidth,a wideband bandpass filter with adjustable center frequency is designed.The output frequency range is from 2290 MHz to 2530 MHz,and the stopband rejection is not less than 50 dB,system output power is from-21.37 dBm to-20.09 dBm,phase noise: <-60dBc/Hz@1kHz;<-110dBc/Hz@1MHz.Finally,the bandwidth,stopband rejection and insertion loss of the bandpass filter were tested and compared with the post simulation of ADS.Through the analysis,it is found that the bandwidth of the narrow bandpass filter is 65 MHz,and the bandwidth of the adjustable broadband bandpass filter is 240 MHz.The power,phase noise,spurs,and output frequency accuracy of the system output,the mixing output,and the separate outputs of the DDS and PLL are tested and analyzed by spectrum analyzer.The test results show that the designed two different bandpass filters filter the spurs after mixing and achieve the preset goal.