Research Of High Linearity And Compact Tunable Bandpass Filter

With development of modern communication system, the tunable filter with high-performance is extraordinary needed. The RF Tunable Filters based on wide-tuning coupling method and mixed-cross coupling theories are thoroughly studied in the thesis. Based on the high selectivity and wide tuning range, the performance of linearity also has been required to improve. The tuning component and magnetic inducor were discussed to improve linearity characteristics. The compact tunable filer with high selectivity, wide tuning range and high linearity is suitable for many wireless systems.Firstly, a circuit mode of tunable filter is researched. Based on Chebyshev and coupling resonator theory, a topological structure of the tunable filter with wide tuning range is realized by properly introducing the appropriate coupling structure. In order to improve the selectivity of the tunable filter, the mixed-cross theory is introduced to the circuit of tunable filter. The 300% tuning range and 4% relative bandwidth are achieved.Secondly, The main influence factor of the linearity is also discussed and the method of cancellation technique and power back off is given to enhance the intermodulation performance. The performance of linearity is a critical factor in shortwave frenquency. The intermodulation of system is increased by the magnetic inductor and tuning varactor. The performance of intermodulation is analyzed and discussed, and the OIP3 and OIP2 of the shortwave tunable filter achieved 30 d Bm and 70 d Bm.Finallly, two compact tunable fitler with small size and high frequency selectivity based on the LTCC techniques are proposed. The wide tuning range is realized by properly introducing the axial coupling structure. Five transmission zeros are obtained near the passband for enhancing frequency selectivity by introducing mixed electric/magnetic(E/M) coupling between the source/load and the taped inductor. The back to back varactor diodes are adopted to improve linearity characteristics. Based on LTCC technology and multilayer structure, the tunable bandpass filter has a very compact size and is suitable for many compact wireless systems. A compact combline tunable filter and a tunable wide bandstop filter(TBSF) with the stepped impedance resonator(SIR) have been realized. By using folded microstrip line and LTCC technology, the size of combline filter only is 5.9*3.5*0.6mm3. Based on a dual-mode resonator and mixed coupling technique, a compact dual-mode TBSF with controllable bandwidth is researched. The compact TBSF can be used to restrain the strong interference signal to improve the sensitivity of communication system.