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Sathyajit - Compact Dual Band Bandstop filters using...

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Compact Dual Band Bandstop filters using Stepped Impedance Resonators Sathyajit B M.Tech,CEDT IISc,Bangalore Introduction: Bandstop filters are used to reject unwanted frequency components. Mixers and Power amplifiers will have double sideband spectral regrowth due to intermodulation and distortion. Dual band reject filters can be used to reject these frequencies. Another added advantage is the low pass band insertion loss when compared to the bandpass filters, because the resonators will resonate in the stopband than in the passband. There are two popular appropaches to implement the dual band response : one which uses Zolotarev function and the other which uses Chebyshev or Maximally flat function with simple frequency variable transformations. Topologies for single band reject filter is discussed in [2]. Parallel-connected different-length open stubs have recently been presented as composite shunt resonators to obtain dual-band performance [4]. However, further size reduction that is achievable by applying stepped-impedance resonators (SIRs) [5] ,[6] in filter design. Circuit synthesis for the DBBSF(Dual band band reject filter) topology used here is presented in [3] and the transformation from Low pass filter to a band reject filter and the equivalent circuit for Dual band response is explained. This report presents the design of DBBSF whose center frequency and bandwidth is controllable. This topology acheives size reduction by using single SIR instead of two parallel connected stubs. The design equations are presented here. A DBBSF with center frequencies 2.45Ghz and 5.2Ghz is designed , fabricated and results are presented. Synthesis of the DBBSF : Fig 1: Schematic of DBBSF of order 2 using SIR's
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Fig 2: Equivalent Circuit for Fig 1 using series LC resonators and a J inverter. DBBSF's have two parallel connected different length open stubs which are quarter wavelength long at the respective midband frequencies. A topology that replaces these parallel connected stubs with SIR's is shown here in Fig 1. Fig 2 shows the equivalent circuit of Fig 1 in which the composite shunt resonator structure is used to get the two antiresonance frequencies. At the first resonant frequency when series resonator formed by L1 and C1 resonate , series resonator formed by L2 and C2 behaves as an open circuit as their impedance is much larger than that offered by the former. At the second stop band
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