Research And Design Of Multi-band Balanced Filter

With the development of social economy,communication technologies such as WiFi,GPS,3G,4G and even 5G have played an increasingly important role in our daily lives.In order to meet the needs of different communication services,the RF front-end needs to work in multiple frequency bands.However,single-passband filters are unable to meet this need,thus creating a real need for multi-pass bandpass filters.At the same time,with the rapid development of integrated circuits,various electrical noises are becoming more and more serious.More and more RF front-end devices such as amplifiers and mixers and antennas are beginning to adopt differential forms to suppress circuit noise.However,the conventional unbalanced filter inevitably adopts a balun structure when interconnecting the above differential devices,which will increase the size of the entire circuit,and is disadvantageous for integration of the entire system.A balanced filter can effectively solve this problem.In addition,the balanced filter has the advantage is that it can be constructed as a fully balanced RF front-end with other differential RF front-end devices such as differential antennas,effectively improving the signal-to-noise ratio of the entire system and reducing signal transmission losses.In order to realize multi-band and suppression of circuit noise in a single filter,this paper studies the implementation method of multi-band balanced filter based on planar microstrip line structure.The main innovations of this paper are:(1)A three-passband balanced filter with multi-drop section loading with independent operating frequency and bandwidth is proposed.By loading the branch near the voltage zero point,the frequency is independently controllable,and the branch loading position is nudged,and a new coupling pitch is generated without significantly affecting the operating frequency,thereby realizing independent controllability of the working bandwidth.At the same time,the asymmetric branch is loaded in the middle of the whole circuit,and good common mode rejection is achieved without affecting the differential mode response.The final simulation results show that this structure has a good differential common mode response.(2)Another three-way balanced filter is designed using the ?/4 SIR structure.Compared with the multi-band balanced three-way belt balance structure,the operating frequency of the three-way band balanced filter using the SIR structure is easier to control,and the adjustment range of the working bandwidth is larger.At the same time,due to the introduction of the hybrid electromagnetic coupling structure,two transmission zero points are introduced between the second and third passbands,which effectively improves the selectivity of the passband.(3)Using the superconducting material,the circuit in(2)is modified into a four-pass balanced filter.Compared with conventional materials,the insertion loss of the circuit after the introduction of superconducting technology is smaller,and the narrow band effect of the pass band is also more excellent due to the higher Q value of the material itself.While retaining the original hybrid electromagnetic coupling structure,the source load coupling is introduced,which can make the circuit generate more zero points and expand the bandwidth of the stop band.