Research On Voltage-tunable Microwave Bandpass Filter

The voltage-tunable microwave filter is widely used in microwave systems such as communication,radar,and electronic countermeasure equipment,due to its capability of adjusting the center frequency,bandwidth,and out-of-band suppression by controlling the bias voltage of its electronic device.It has become one of the important research directions in the current microwave field for the fast conversion of different functions.As an indispensable part of the UWB integrated system,the voltage-tunable microwave filter can quickly select the required channel,suppress the interference signal,and meet the current development trend of system integration and miniaturization.So it has extremely high application value and lays the foundation for the development of ultra-wideband integration technology.In this paper,the resonance characteristic,coupling principle and tuning mechanism of different varactor-loaded structures are studied based on the fast response of varactor diode.And different methods for improving the filter performance are proposed based on the theory of multi-mode resonator and cross-coupling.This paper proposes several methods for improving out-of-band suppression by analyzing the coupling principle of different feed structures and utilizing the theory of electromagnetic coupling and phase delay.The current scarce high frequency tunable filter is studied in this paper,which expands the achievable adjustable frequency band and makes it possible to use in ultrawideband.The main research work of this paper can be summarized as follows:1.A 14.25~17.61 GHz microstrip voltage-tuned narrowband filter based on varactor tuning technology is proposed,which breaks the frequency band of currently available microstrip tunable filter based on varactor tuning.The characteristics of several microstrip tunable resonators are studied and the corresponding design formulas are deduced.Then a miniaturized filter with tunable frequency,a center frequency and bandwidth tunable filter with multiple transmission zeros,and a tunable filter with constant bandwidth are designed respectively based on the novel and improved combline structures proposed in this paper.They have the characteristics of small size,wide tuning range,good passband transmission and good stopband attenuation.Based on the openloop structure and cross-coupling theory,an open-loop tunable filter with high selectivity is proposed,whose adjustable range is greater than 50% and insertion loss is less than 4.8d B,and transmission zeros on both sides of its passband are greater than 25 d B.2.Through the deeply study of the resonance characteristic of SIR(Stepped Impedance Resonators)and SLR(Stub-Loaded Resonator),the method for flexibly adjusting the resonance mode and reducing the volume is proposed.The compact adjustable triple-mode filter designed here has a wide upper stopband,and its-20 d B rejection extends to 6.5 octaves.A compact adjustable dual-mode filter is proposed based on the odd-even mode theory and the coupling between source and load.Its outof-band characteristic is improved by means of the introduction of the transmission zeros on both sides of the passband.And the transmission zero of upper stopband is always greater than 35 d B.The adjustable frequency range of 0.846~1.21 GHz and adjustable bandwidth range of 80~194MHz are realized.3.The high-frequency tunable filters are designed based on SIW(Substrate Integrated Waveguide)and varactor diode.Their equivalent circuits,resonance characteristic and the method that varactor diode coupling to resonators are studied.The SIW tunable filter designed here solves the design problem of the tunable filter in the microwave high frequency band.In order to adapt to the miniaturization trend of current system,the idea that applying slow-wave substrate integrated waveguide(SW-SIW)and quarter-mode substrate integrated waveguide(QM-SIW)structures to tunable filter is proposed.The SW-SIW tunable filter with 62.3% reduction in cavity size and QM-SWSIW tunable filter with 73.3% reduction in cavity size are realized,which proves the correctness of the design concept.