The Research Of Bandpass Filter In Hybrid Synthesizers

Frequency synthesizer is viewed as a key component in modern communication system,its characteristics have great influence on the performance and quality of the whole system.With these communication protocols are more and more complex,the performance requirements of frequency synthesizer are higher and higher.Thus,the single PLL or DDS frequency synthesis technology can’t meet the frequency source requirement of high-end technology,and then the PLL+DDS hybrid synthesis technology is appeared.As an important part of hybrid synthesis,Band-pass filter has a major effect on the output spectrum quality of the hybrid synthesis.Therefore,this article will do a research of band-pass filter in hybrid frequency synthesis,and this paper’s works are shown as follows:(1)In this paper,it introduces the theory of DDS and PLL frequency synthesis,and their performance of output frequency,phase noise characteristics and spur are simulated,respectively.The cause,distribution characteristics and suppression method of phase noise and spur are studied.There are three types of PLL+DDS hybrid synthesis,including the DDS driven PLL,DDS+PLL mixing in the inside of feed-back loop and DDS+PLL mixing in the outside of feed-back loop.Then a contrast analysis on the advantages and disadvantages of them are made.Some requirements of band-pass filter are put forward on the basis of hybrid frequency synthesis of DDS+PLL mixing in the outside of feed-back loop.(2)According to the difference of hybrid-frequency-synthesis’ output frequency range,simulation and analysis are made on the different kinds of band-pass filter and their advantages and disadvantages are compared.For KHz output frequency band,digital band-pass frequency domain filter is a good choice,looking for the influence of the maximal spur’s amplitude and threshold factors on filtering effect;For the under 500 MHz output frequency band,there is a theoretical derivation of a lumped element filter circuit structure which has constant bandwidth and adjustable center frequency,Then simulation and analysis are made on the performance-index’s change law of this filter.For the over 500 MHz including GHz output frequency band,the comb-line filter’s parameters are calculated by the formula design method which has variable bandwidth and center frequency adjustable.Furthermore,simulation and analysis are made on the performance-index’s change law of this filter;Dielectric filter is simulated which is applied to the GHz frequency band and its center frequency can’t be adjusted,exploring the difference of pass-band and loss characteristics between dielectric filter and comb-line filter.