Spurious suppression of dielectric filters in practical wireless systems

With the rapid expansion of the wireless communication systems, the demand for microwave filters with strict specifications has increased in recent years. Dielectric filters operating at the TE₀₁ mode are being used to achieve high-Q filter functions. However, dielectric filters suffer from poor spurious response, i.e. the undesired higher order resonant frequencies exist in the vicinity of the operating frequency. Different techniques have been used in the literature to suppress these spurious modes. This paper presents a new practical design that combines a combline filter with the main dielectric filter to suppress its undesired spurious frequencies. The presented integrated solution is cost effective, and has minimal effect on the complexity of the system. This makes it an attractive solution for mass production of microwave filters for base station application. To exhibit the proposed concept, a four-pole combline filter is integrated with a six-pole TE₀₁ mode dielectric filter that is used in a WiMax system. The measured results show the usefulness of the proposed method. The practical considerations that make this method suitable for mass production and the tuning procedure of the integrated ten-pole filter are also discussed.