Ceramic waveguide bandpass filters with spurious modes suppression

An investigation of various ceramic bandpass filters with an improved spurious performance for use in cellular base station filtering applications is presented in this thesis. Monolithic integrated ceramic bandpass filters offer more than 50% size reduction compared to air-filled coaxial resonator filters with the same unloaded Q-factor. However, the stopband performance of these filters is deteriorated by spurious frequencies of the fundamental mode and higher order modes. The probable solution to reducing the effects of these undesired modes is to add a low-pass filter or band-stop filter at the expense of higher in-band losses and bulky volumes. In this work, multiple geometrical design techniques are explored to achieve the optimum out of band performance without any need for a low-pass filter.
The improvement in spurious performance of Chebyshev ceramic bandpass waveguide filters is explored. In particular, its design solution aimed to improve the stopband attenuation for these filters. The design of the Chebyshev monolithic ceramic bandpass filter is reviewed, and some realizations are proposed and compared with the filter.
The sixth order Chebyshev ceramic bandpass filters with posts were designed with improved spurious performance. The input/output couplings are realized through the use of a coaxial cable placed at the center of the external resonator. The inter-resonator couplings are achieved by placing various metal plated through-holes in the broad dimension of a waveguide.
The broad dimension of integrated ceramic waveguide resonators can be utilized as an extra degree of freedom that can be integrated into filter design procedure and a better spurious performance is achieved by mixing resonators of non-uniform widths. Chebyshev ceramic bandpass filters are designed with two and three non-uniform width resonators and significant improvement has been achieved in the stopband performance of the filters.
Other solutions involve the mixing of resonators with posts and non-uniform width resonator. The six order ceramic waveguide bandpass filters are simulated and fabricated in an air filled waveguide with tuning screws. Metal tuning screws are included to overcome mechanical discrepancies and imperfections.
The stepped impedance resonators were previously applied to both planar, coaxial and air-filled rectangular waveguide filters. Here, for the first time, their use has been extended to monolithic integrated ceramic waveguide filters, accomplishing an exceptional spurious free stopband bandwidth for the filters.
Finally, sixth order ceramic loaded waveguide filters were designed and fabricated with ridge and non-uniform width ceramic blocks. Their top and bottom surfaces are metalized through the silver paint with the conductivity of 4 x 10⁷ s/m. Inductive irises are used for inter-resonator coupling where the coaxial probe excites the external resonators of the filter and the excellent stopband attenuation of up to 2.45 *f_o is achieved.