Design Of Tunable Microwave Filter Based On BST Thin Film

The development of frequency hopping communication system in broadbandcommunication makes the widely research on intelligent RF front-end. And as a corecomponent, the tunable filter becomes the focus of the study. Due to the ferroelectricfilm capacitor which is used as tuning elements of the tunable filter has a fast responsespeed, small loss at high frequency, high adjustable rate and low production cost, thistype of filter has become a hot spot of military research and universities at home andabroad. Therefore, this article designs ferroelectric tunable filters basing on the filtertheory and produced a physical filter for validation. Main work and conclusions of thisresearch are as follows:1. A tapped comb-line micro-strip band-pass tunable filter has been designed at acenter frequency of870MHz. The bias isolation of this filter was designed withBST-bias capacitor and a high resistance line between resonators with width of20μm.Compare to the traditional design with bias resistors, this design can reduce bias tees orsimplify the bias process by load the DC signal on signal line directly. The results EMsimulation as: the comb-line filter has a bandwidth of9.2%; the center frequencyadjustable rate is14%at the tuning rate of36%and the return loss is better than15dB;the insertion loss is4.9dB as the dielectric loss of0.02.2. A half wavelength open ring band-pass tunable filter has been designed at acenter frequency of7.8GHz. The Bias isolation of this filter was designed with highresistance thin strips between resonators with width of8μm and the thickness of80nm.And high resistance lines to ground with width of8μm and length of quarter-wave. Theresults EM simulation as: The open loop filter has a bandwidth of8.2%；the centerfrequency adjustable rate is12.8%at the tuning rate of33%and the return loss is betterthan12dB; the insertion loss is4.65dB as the dielectric loss of0.02.3. Effects of fabrication errors on the device performance were simulated as: theupper electrode with a thickness range of2~3μm makes no difference to filter; theabsolute error more than2μ m of unilateral lithography and etching process, wouldeasily cause a short in BST capacitors; the deviation of dielectric properties of BST film capacitance would increase insertion loss and reflection.4. The prototype device of ferroelectric tunable filter has been completed with thecompletely manufacturing process. The center frequency is1.42GHz, and the insertionloss is about10dB; the frequency can adjust40MHz with DC bias in30V; the feasibilityof the structure and tunable characteristic has been verified.