Design Of Independently Tunable Microstrip Dual-band Filter

Now, wireless communication technology develops rapidly, multiple wireless communication standards coexist. The research on the multi-band of communication device is also been widely studied. And multiband microwave components as fundamental parts are important. Especially, the multiband bandpass filters with tunable center frequency are getting more and more attention owing to their potential to meet the present requirements of system size and complexity, because a single tunable multi-band filter can support multiple information channels and minimum the number of microwave filters.This dissertation mainly focuses on the two kinds of microstrip resonators and DGS structure. And we use them and varactors to design two independently tunable dual-band filters. The main contents of the concrete work are as follows:(1)This paper introduces the electromagnetic coupling structure design method of the tunable filter. We can easily determine the initial value of structure size through studying the external quality factor and the coupling coefficient of filter.(2)We propose a novel asymmetric λ/4 resonator pair with shared via-hole ground to realize dual-band response, and two separated passbands are independently generated by the two λ/4 resonators with different electric lengths. A prototype tunable dual-band filter with chebyshev response is designed and fabricated. The measured results show that the first passband varies in a frequency range from 0.88 GHz to 1.12 GHz with the 3-dB fractional bandwidth of 5.1%–6.4%, while the second passband can be tuned from 1.5GHz to 1.81 GHz with the 3-dB fractional bandwidth of 5.4%–6.4%.(3)Based on the short stub-loaded resonator, odd-even modes are analyzed and studied. When the length of the short circuit branch section is very small, odd-even modes are approximately equal, and couple each other to form a single resonant frequency. Two different short stub-loaded resonators with common input and output ports are used to design at unable dual-band filter. The special coupling structure can introduce transmission zeros between passbands and improve the selectivity of the filter. The measured results show that the first passband varies in a frequency range from 0.81 GHz to 0.95 GHz with the 3-d B fractional bandwidth of 4.2%–5%, while the second passband can be tuned from 1.51 GHz to 1.79 GHz with the 3-dB fractional bandwidth of 6.8%–8%. Especially, transmission zeros vary with operating frequencies, and the filter obtain high out-of-band rejection in the whole tuning process. And different sizes of DGS structures are respectively added to input and output ports in the previous tunable dual-band filter. The measured results show that the higher-order modes are successfully restrained with slight influence on operating frequencies.