Design Of Frequency Tunable Balun And Bandpass Filter

Baluns and bandpass filters are important devices for frequency allocation and selection,respectively,and play an important role in RF front-end applications.At present,with the rapid development of wireless communication technology,stricter requirements are placed on baluns and bandpass filters,including high performance,high reliability,miniaturization and low cost.The microstrip baluns and bandpass filters have been widely developed due to their planar structure,small size,light weight,high reliability and low cost.In recent years,with the continuous change of wireless communication standards,the demand for reconfigurable communication systems that can achieve multi-band coverage is becoming more and more urgent.Therefore,it is of great significance to study the frequency tunable balun and bandpass filter.Supported by the National Naturel Science Foundation of China(Foundation number:61401056),the frequency tunable balun and bandpass filter are respectively studied.Details are as follow:(1)A frequency tunable balun is proposed.The balun is based on varactors loaded coupled lines.By tuning the capacitances of the varactors,wide frequency tuning range is obtained.Moreover,good impedance match are maintained during the tuning.A detailed theoretical analysis using the signal flow diagram and the even-odd mode method is presented,and the design formulas is derived.The circuit is simulated with ADS,and a prototype with a tunable frequency of 0.875?2.0GHz(78.3%)is fabricated and tested.The measured results show that more than 32dB input return loss,less than 4.3dB insertion loss,1.5°phase difference and 0.48dB amplitude imbalance are obtained at all tuning center frequencies.The balun has a simple structure and easy to produce.Due to the above reasons,it may has practical application value.(2)A frequency tunable balun with an isolation circuit is proposed,which is composed of varactors loaded coupled lines.The feasibility of the design is verified by detailed theoretical analysis and formulas.The circuit is simulated with ADS,and a prototype with a tunable frequency of 1.0?2.0GHz(66.7%)is fabricated and tested.The measured results show that more than 21dB input return loss,less than 4.7dB insertion loss,1.80 phase difference and 0.43dB amplitude imbalance are obtained at all tuning center frequencies.Moreover,great output matching(|S22|<-22dB,|S33|<-22dB)and isolation(|S23|<-25 dB)are obtained at each tuning center frequency.The design meets the requirements.(3)A frequency tunable bandpass filter is proposed.The bandpass filter is based on trans-directional coupler and short-circuited stubs.A detailed theoretical analysis and formulas derivation are proposed for the design.The circuit is simulated with ADS,and a prototype with a tunable frequency of 1.0?1.6GHz(46.2%)is fabricated and tested.The measured results show that more than 17dB return loss,less than 1.4dB insertion loss,the 3dB fractional bandwidth of 43.2?50%and rectangular coefficient(BW10dB/BW3dB)of 1.28?1.44 are obtained at all tuning center frequencies.The design has good performance.