Selective Six-Pole Microstrip Bandpass Filters for 4G Applications

Six-pole microstrip bandpass filters composed of square C-shaped resonators are designed for 4G applications at 2.6 GHz. To improve the filters selectivity around the passband, two pairs of transmission zeros are introduced by coupling of multiple resonators. The filters, which have different topologies, contain two cross couplings of opposite types in order to obtain a very sharp skirt. The simulation and measurement results show very good reflection and transmission characteristics. The proposed filters are synthesized using AWR® and simulated using CST® simulation tools. The simulated reflection coefficients are better than -13dB whereas the simulated transmission coefficients are better than -1 dB. In order to validate the simulations, one filter has been manufactured and measured. The simulated and measured results are in good agreement. The measured reflection and transmission coefficients are better than -12 dB and -4 dB respectively. The simulated and measured 3dB bandwidths are 80MHz and 60MHz respectively. A small shift in the centre frequency is obtained which is attributed to mechanical tolerances and tolerances of the thickness and dielectric constant of the substrate used.