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MEMS tuned resonators

We present a concept of the fast tuneable dielectric resonator for applications in mobile communication systems. The tuning approach is based on the interaction between two resonant modes: a cylindrical dielectric resonator of TE01d mode and a planar microwave slotline resonator. Variation of the coupling between two resonator modes leads to change of the rf electro-magnetic field distribution in the resonator assembly and as the consequence to variation of the resonance frequency of the resonator. In order to control the intermodal coupling we apply fast microelectromechanical (MEMS) or piezoelectric switches.

Based on this concept discrete tuning by 5 MHz in 0.25 MHz frequency steps was demonstrated for a test resonator at 2 GHz. The unloaded quality factor is about 10.000 and the measured switching time is about one millisecond.

Within our tuning approach and resonator design a substrate with 4 radial slotlines is symmetrically arranged above a cylindrical dielectric resonator of TE01d mode. One end of each slotline is short circuited while another end is loaded by a MEMS switch. For the case of a switch off-state, each of the 4 slotlines represent a capacitively loaded quarter wave slotline resonator which possesses a strong magnetic field component in radial direction providing strong coupling to the resonant fields of the TE01d mode.

In this case, due to interaction between the dielectric resonator mode and the slotline resonator mode the stored electromagnetic energy is distributed between the DR and the slotlines. This leads to an increase of the effective size of the resonator and, as a consequence, to a decrease of the resonant frequency of the TE01d mode. Upon closing one switch, the corresponding quarter wave resonator changes into a halfwave resonator. Due to the sign change of magnetic field in the middle of the slotline, the coupling between slotline mode and TE01d mode is much weaker. In this case there is almost no intermodal coupling and the resonance frequency of the TE01d mode remains almost unaffected by the presence of the slotlines.

In order to evaluate the expected tuning range and to choose an optimum slotline configuration, numerical field simulations have been performed employing the commercial code “CST Microwave Studio”. For our resonator design the spatial distribution of resonant fields were calculated for the capacitive (open) and resistive (closed) state of the MEMS for the case of 4 symmetrically arranged slotlines. As expected, the field distribution differs substantially. There is an obvious excitation of the slotlines with open end. For the resistive state, the field is localized primarily inside the dielectric resonator, therefore the effects of slotlines and MEMS are not significant and frequency is about that of the non-perturbed dielectric resonator. Variation of the frequency determines the tuning range, which is about 3% (~60MHz) at quite high quality factors up to 7000.

The principle of tuning was proven by an experimental setup based on commercial piezoelectric bimorph actuators. Our setup is based on a cylindrical dielectric resonator (er = 28 - BZT ceramic) with TE01d mode at about f = 1.86 GHz is arranged in a copper cavity. A 0.5 mm thick copper disc with radial slots was assembled symmetrically above the dielectric resonator. As MEMS equivalents, four metallized bimorph piezoelectric actuators control the slotlines states and consequently, the coupling between the dielectric resonator and the slotlines. A large tuning of the resonance frequency is observed with a very challenging values for the figure-of-merit for tuneable resonators being about 160.

One of the interesting results, which we found, is that there is no significant intermodal coupling between different slotline resonator modes. Therefore, each of the slotlines can be considered as an independent tuning element. This behaviour can be used as digital frequency tuning. To demonstrate the principle of the digital tuning within a frequency range of 5 MHz in 0.25MHz frequency steps we applied 8 bulk slotlines for a test resonator at 2GHz. The unloaded quality factor was found to be between 10.000 and 12.000 depending on the switching state of this digital tunable resonator. It was shown that a discrete piezoactuator based MEMS tuning concept for dielectric resonators is quite challenging and has a great potential to beat the performance of semiconductor based tuning techniques.

Optimisation of the planar resonator structures in order to reach larger tuneability while keeping the highest quality factor is subject of our future activities.

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