Study On The Design Of Elliptic Bandpass Filters Constructed On Multilayer Substrates

Miniaturization and high-performance are always the goals in microwave filters design. There are usually two choices in the realization of filters, one is using distributed-elements and the other one is using lumped-elements. The filters designed by using distributed-elements usually have excellent performance but large size while ones designed by using lumped-elements feature small size but average performance. Using the technique of multilayer substrate could make the size of filters smaller, but the parasitic capacitors and inductors in multilayered structure is a big problem. To solve this, choosing elliptic-function filter as the circuit schematic may be an effective way. In the elliptic–function filter schematic, the inductors and capacitors usually appear in pairs, which could make the parasitic capacitors and inductors absorbed by designing appropriate circuit schematic. What is more, the elliptic-function filter, which is famous for its high selectivity in frequency, is the best choice when designing a filter with high performance. On this basis, elliptic function bandpass filter constructed on multilayer substrates is fully discussed and the contents are listed as follows:(1)Starting from the third-order elliptic bandpass filter schematic, a new bandpass filter schematic is proposed by using Norton’s transformation. This novel schematic effectively solve the problem that the inductances might be too large to realize in multilayer structure when designing a specific filter with high out-band rejection using conventional elliptic-function filter. The influence on the performance of the filter from the parameters has been investigated by using the electromagenetic simulation software named ADS.(2)Based on the deep analysis of the capacitors and inductors in multilayer structure, the idea for designing the whole physical layout starting from the capacitors is proposed. Finally, a filter physical model with logical layout is designed, in which a novel coupling capacitor structure is applied creatively. Using this design method and the novel coupling capacitor structure could reduce the complexity in the process of tunning.For experimental verification, a practical example of the filter design is given, corresponding physical model is fabricated and measured. After data analysis, the measured results agree well with the theoretical simulation results, which satisfy the design specifications of high selectivity in frequency and high out-band rejection.