Fast resonators for mobile communications
Base stations emitting mobile telecommunication frequencies at approximately 2 GigaHertz (GHz) are obliged under strict EU regulations to satisfy specific demands posed on their filter performance. To meet current but also future EU requirements the TUF project has developed innovative dielectric resonator filters. These new resonators are characterised by extremely low dielectric losses and can also be used in other scientific fields for accurate measurements of total radiation power (bolometry). The tuning is realised with the aid of two coupled resonant modes, a cylindrical dielectric resonator with resonance at the TE01d mode and a planar microwave slotline resonator. The resonance frequency of the resulting resonator assembly changes following the variation of the coupling of the two modes. This intermodal coupling is controlled with fast microelectromechanical (MEMS) or piezoelectric switches. Such laboratory developed and patent protected resonator assembly with working frequency at 2GHz is capable of tuning by 5MHz in 0.25MHz frequency steps. Its unloaded quality factor is about 10.000 and its measured switching time is about one millisecond. Variation of the slotline resonator configuration alters the intermodal coupling and consequently affects the tuning range. An arrangement of the cylindrical resonator with four radial resonators symmetrically placed below it resulted in minute intermodal coupling and left the resonance frequency of the cylindrical resonator practically unaffected. With numerical field simulations project partners searched for the optimum slotline configuration. For the case of four symmetrically arranged slotlines on a plane, it was found that their presence in the assembly did not produce any significant effect and the frequency remained the initial cylindrical resonance frequency. By replacing the MEMS with commercial piezoelectric bimorph actuators, successful tuning has been achieved. The above very promising results oblige project partners to further optimise planar resonator structures. They are currently seeking to reach larger tuneability without dispensing with the high quality factor already achieved.
