Research On Cross-coupled Tunable Bandpass Filters With Constant Absolute Bandwidth

With the continuous development of multi-band,frequency conversion and software assisted wireless communication system,the performance requirements which belong to RF front-end components are improving continuously.Filter,the core device of RF front-end electromagnetic signal channelization,plays crucial role in the whole RF front-end signal transceiver function.The planar microstrip tunable filter realizes the controllability of the filter frequency by adding tunable devices,so that the filter can filter at different frequencies,which reduces the number of components used and the cost to a certain extent,hence the way for overall adjust and control RF circuit are more.The CT-type microstrip filter can construct a cross-coupling transmission channel with multiple coupling paths,resulting in an asymmetric frequency response,and the transmission zeros are mostly on the single-frequency side of the passband.Compared with the design of the CQ-type filter,it can save area,reduce cost,promote the miniaturization of the filter,reduce the cost of a communication system that only requires high frequency selectivity on one side of the passband.A filter with constant bandwidth can reduce the effect of noise on the high-frequency side of the passband effectively,while improving the stability of the overall signal selection process.The main research content of this thesis can be divided into two parts:Firstly,a novel CT-type pure electric cross-coupling resonator unit is proposed based on the electrical coupling distribution of the half-wavelength resonator loaded with variable capacitance,and the designability of this resonator unit is verified.A third-order pure electric cross-coupling filter topology is constructed based on this proposed CT-type resonant unit.The target characteristic curve of the third-order filter is obtained by optimization,and the regulation method of pure electric coupling between different resonators and realization mechanism are given,and the specific coupling coefficient derivation process is given.A single transmission zero on the high-frequency side of the passband is introduced.The results after fabrication show that the center tuning frequency of this CT-type third-order pure electric cross-coupling tunable filter with constant absolute bandwidth is in the range of1.47～1.86 GHz,the 3d B absolute bandwidth range is 108.75±2.05 MHz,and the attenuation coefficient ranges from 3d B to 25 d B is 327.32～623.57 d B/GHz.The measured intermodulation variation range of the third-order intermodulation point is 4.73～24.59 d Bm.Secondly,a half-wavelength magnetic coupling resonator structure is introduced into the designed CT-type pure electric cross-coupling resonator unit based on the magnetic coupling distribution of the half-wavelength resonator loaded with variable capacitance,which aims to construct a fifth-order cross-coupling filter topology.The electromagnetic coupling control method and implementation mechanism of the high-order filter are explored.The fifth-order filter topology introduces a single transmission zero point on the high-frequency side of the passband.The results of fabricated filter shows: the center frequency can be tunable within a range of 1.45 GHz to 1.92 GHz,the range of 3d B absolute bandwidth is maintained at101.9±1.3MHz,the attenuation coefficient ranges from 3d B to 30 d B is442.75～843.59 d B/GHz,and the intermodulation range of the third-order intermodulation point are 3.85～23.34 d Bm with the change of the bias voltage.