Multilayer microwave structures using thick-film technology

Abstract

Multilayer techniques, in conjunction with thick-film technology have been applied to the design and fabrication of several multilayer microwave structures to achieve the low cost and high performance goals set by modern microwave circuits and systems. To provide accurate material parameters for the design of multilayer thick-film components, a novel slit cavity resonator method has been developed that enables the relative permittivity and loss tangent of dielectric samples to be measured easily, and with high accuracy. A particular feature of this method is that it can be used to measure thick-film samples that are normally only available in relatively thin layers in a two-layer format. Rigorous electromagnetic analysis on a slit cavity has been performed that accounts for the effect of the fringing fields and the radiation from the slits. The method has been verified through measurement on several thick-film materials over X-band. Both the analytical methods and the fabrication techniques for multilayer microwave microstrip structures are presented. Several multilayer thick-film microstrip line test structures have been designed and characterised, and these provide a basic database for the design of multilayer microstrip components. A new design procedure for the multilayer end-coupled filter has been developed that enables the designer to arrive at the physical dimensions of the multilayer structure based on the filter specification. This design technique is effective as it combines the accuracy of electromagnetic (EM) analysis and the efficiency of circuit simulation. The multilayer gap, which is the most critical element of multilayer end-coupled filters, has been characterised using EM analysis and the data is incorporated into a circuit simulator. Measured and simulated results are presented that verify the new design technique. A 40% bandwidth has been achieved experimentally, which shows a very significant improvement over conventional single layer structures, where the bandwidth achievable is normally less than 5%. Novel, octave band DC blocks have been designed, fabricated and tested using a new multilayer format. The tight coupling required between the coupled lines in this component was realized by overlapping these lines in a multilayer structure. Very good agreement was obtained between measured and simulated data. The multilayer approach was also applied to the design of coupled line bandpass filters where a measured 80% bandwidth was achieved. For the first time, the properties of multilayer coupled lines using a range of different thick-film dielectrics are examined using their coupled-mode parameters. Design curves for multilayer coupled lines are obtained, that provide important information on the design of multilayer directional couplers. A practical design strategy for multilayer directional couplers is developed, which overcomes the problem of excessive computation that is normally associated with the electromagnetic optimization of multilayer circuit designs. The methodology has been verified through the design and measurement of wide bandwidth 2dB and 3dB directional couplers that were fabricated using multilayer, thick-film technology. New techniques for the design and fabrication of multilayer microwave thick-film components have thus been established, both theoretically and through practical circuit fabrication and measurement.