MICROPACKAGED MAGNETOSTATIC WAVE RESONATORS IN MICROSTRIP AND CPW CONFIGURATIONS
R. MARCELLI, G. SAJIN , A. CISMARU

Introduction
Microwave sources having broadband characteristics and high spectral purity are presently required for modern space and ground communication systems. The magnetostatic wave (MSW) technology is well known for providing frequency tunable filters and oscillators for linear and nonlinear microwave signal processing. Planar microwave oscillators and filters based on MSW excitation in epitaxial garnet films have been extensively studied in the past. Actually, they are characterized by reasonable values of the external quality factor Qext (oscillators: between 1 000 and 3 000, resonators: more than 3 000) and broadband tuneability. Furthermore, the possibility to use a planar geometry allows the resonator to be easily integrated in complex structures, overcoming in that way the well known problems introduced by the bulk yttrium iron garnet (YIG) sphere devices. A further qualification factor for magnetic film based microwave sources is the low phase noise, down to –105 dBc/Hz @ 20 kHz at 10 GHz for the planar configuration, comparable with the most part of the commercially available YIG sphere sources.
The micromachining technology was introduced in the field of the RF, microwave and millimeter wave applications at the beginning of the 90's, to improve functionalities, integration and packaging of conventional devices. Moreover, it has been demonstrated the enhancement of the electrical performances of switches, resonators, filters and antennas based on this novel technological solution. In this work, micromachining is used as an alternative technique for obtaining an electrical matching of a YIG straight edge resonator (SER) as a potential candidate for the feedback section of microwave sources to be manufactured on silicon wafers. This has been minded in agreement with the most part of the requirements for low cost and high performances devices to be integrated in an all-silicon environment. For the above reasons, both bulk and micromachined structures have been realized and tested to demonstrate the improvements which can be obtained in terms of frequency tuneability and insertion losses. The obtained results are comparable with those well established in the case of alumina based devices, which are not easy to be integrated as the silicon based ones. MSW resonator configurations on a silicon bulk substrate and on a micromachined silicon membrane, have been characterized and compared by measuring their S-parameters response at different DC magnetic bias fields. The frequency has been swept between 2 GHz and 5 GHz for silicon bulk resonator and between 2 GHz and 6.5 GHz for the resonator supported by the silicon membrane. By using the membrane technological solution, the insertion losses for membrane supported resonators decrease by about 18 dB with respect to the bulk silicon supported one. Moreover, the quality factor measured for these configurations was QL  450, about twice that for silicon bulk supported SER. A further improvement of the above configurations is given by possible micropackaging of the resonator, which allows an easier assembling of the device. Actually, the resonator positioning with respect to the transducers is a critical issue, and on-line adjustments are often required for improving the electrical response. This is often performed during the test of the device, to fix the final mechanical alignement. Moreover, the coupling between SER and transducer is limited by the EM field pattern, and the side coupling does not favour an optimal excitation of the magnetic film based resonator by means of the RF magnetic field. For both purposes, an etched version of the SER device is also proposed. For wideband response of band-pass resonators, a microstrip configuration has some intrinsic limitations, due to an unavoidable cross-talk between input and output. Actually, the microstrips are well separated, but an oscillatory response can be predicted in some frequency ranges. In our case, for instance, the X band seems to be not favoured by the usual dimensions proposed for both microstrips and SERs. In particular, the cross-talk between the two transducers is no more negligible in the middle of the X-band as well as around 22 GHz, where the off-resonance isolation between the input and output ports for the SER is worst. For this reason, a coplanar waveguide (CPW) structure has been designed to have a wide-band response and no cross-talk at high frequencies..