Research On Tunable Filters Based On Evanescent-Mode Cavity

Tunable filters have important applications in many fields such as electronic countermeasure and microwave measurement.The most mature scheme of single tunable filter is based on YIG technology,but this technology still has big volume and high power consumption,not easy for integration.At the same time,many planar tunable filters developed in recent years have the problems of that wide tunable range and high quality factor of these filters are not compatible.Therefore,it is of great value to develop a kind of tunable filter which has wide tunable range,high quality factor and easy for integration.So the goal of this study was to research a new type of tunable filter based on sevanescent-mode cavity.The main works and conclusion are summarized as follows:1.Analysis of the electromagnetic field of evanescent-mode cavity using HFSS electromagnetic simulation software and the influence of parameter on resonator performance was done.Results show that reducing the physical size of the resonator and add dielectric film between tuning capacitor plates are helpful to improve the resonant frequency.However,these methods will bring the deterioration of the quality factor.The final selection of cavity radius is 6.5mm;the loading post height is 3.5mm;the loading post radius is 1mm;the initial loading capacitor plate gap is 10?m.Analysis shows that the frequency tuning range of the resonant cavity is from 2.46 GHz to 5.02 GHz,and Qu of the resonator is 975-1469,as the gap increased from 10 um to 50 um.2.A two-pole tunable bandpass Butterworth filter is designed,when the load capacitance plate gap increased from 5?m to 15?m,Analysis shows that the tuning range of the center frequency is from 1.74 GHz to 2.85 GHz with an insertion loss of 0.86-1.35 dB and a 3-dB fractional bandwidth of 1.89%-2.07%.3.The filter and the resonator were fabricated and measured by a network analyzer.With 100 V of bias voltage,the resonant frequency of the tunable resonator can be tuned between 4.42 and 2.49 GHz and Qu of the resonator is 470–645.the filter is continuously tunable from 3.045 to 3.73 GHz.It has an insertion loss of 8.42–9.87 dB and a 3-dB bandwidth of 20.4–35.6 MHz across the tuning range.