Design of a Compact Microstrip Triple Independently Controlled Pass Bands Filter for GSM, GPS and WiFi Applications

An ultra-compact triple-band bandpass filter based on dual-mode quarter-wave resonator for GSM, GPS, and WiFi applications is presented in this study. The filter consists of three quarter-wave resonators assisting in controlling each passband independently. The first operating frequency band is obtained by the direct-feed resonator which acts as a source to load coupling for the inner two resonators which are operating at higher frequency bands. The outermost resonator is coupled to the inner loaded resonators, thus a pair of transmission zeros between each passband can be excited and hence high-frequency selectivity can be obtained. To miniaturize the overall size of the filter, all resonators are folded and are jointly connected through a common metallic via with the ground. The design has a symmetric structure therefore, even-odd mode analysis method is applied to obtain the three controllable operating frequency bands. The first operating band is centered at 850 MHz which caters for GSM applications, while the second and third frequency bands centered at 1.57 GHz and 2.4 GHz fall in the GPS and WiFi wireless applications. The filter with an ultra-compact size of <inline-formula> <tex-math notation="LaTeX">$0.10\,\,\lambda _{\mathrm {g}} \times 0.09\,\,\lambda _{\mathrm {g}}$ </tex-math></inline-formula> (<inline-formula> <tex-math notation="LaTeX">$0.009\lambda _{\mathrm {g}}^{2}$ </tex-math></inline-formula>) despite feed lines (<inline-formula> <tex-math notation="LaTeX">$\lambda _{\mathrm {g}}$ </tex-math></inline-formula> is the waveguide length centered at 850 MHz) is designed, fabricated and measured for the purpose of validation. Both the simulated and measured results are in good agreement and endorse the design concept.