Design Of Tunable Filters For RF/MW Applications

This thesis presents tunable bandpass filters in the frequency range of 1.5–2.7GHz for RF and microwave applications.The proposed tunable filters are designed for high selectivity,size miniaturization,wideband and constant absolute bandwidth(CABW)characteristics while maintaining low insertion loss.The size miniaturization is obtained by using new novel microstrip staircase resonators.The high selectivity is achieved by introducing two controllable transmission zeros on both sides of the passbands.The wideband and the constant bandwidth characteristics have been achieved using novel miniaturized staircase combline resonators.In the low-loss high selectivity design,firstly,a compact single/dual-band bandpass filter is introduced with an independently configured second passband and high out-of-band rejection level,then a low-loss tunable bandpass filter is presented using the same compact grounded staircase resonators with frequency tuning range of 1.58–1.88GHz,and an additional transmission zeros tuning feature is introduced to give better and enhanced rejection levels from the interfering signals.The measured 30dB/3dB bandwidth is ranging from 1.71 at 1.58 GHz and 2.47 at 1.88 GHz while the measured insertion loss is less than 2 dB across the whole tuning band.In the wideband and constant absolute bandwidth design,novel staircase combline resonators are used to realize highly compact fixed and tunable bandpass filters.The proposed structure is firstly used to design a wideband filter with 62.5%of fractional bandwidth(FBW)on only 6.5×8.5 mm~2 of area.Then a tunable bandpass filter with wide tuning range 1.6-2.7 GHz and a constant absolute bandwidth(360±10)MHz is designed on an area of just 6.7×8.7 mm~2.Furthermore,an additional feature of quality factor control is introduced to offer more enhancement in the filter performance and also to provide a limited bandwidth tuning capability.Generally,and according to our own knowledge,the proposed filters represent state-of-the-art in terms of insertion loss,high selectivity and compact size characteristics in comparison with other reported varactor-based tunable filters.