Area Optimized Thin Film Coupled Inductor Band Pass Filters with Integrated Baluns

Modern commercial telecommunication devices tend to integrate more and more functionality on a single chip, the so called SoC (System on a Chip) approach. This results in ultra-compact and low cost realization. However antenna filters and diplexer are still important parts in the transmit/receive chain for which until now no on-chip solution is possible that meets the requirements. In this work we show how it is possible to realize high quality, ultra compact and low cost filters using IMEC's general purpose RF thin film technology. Narrow band pass filters can be realized in different fashions, but they all consist of coupled resonators. High Q-values of the resonators are required to realize narrow band, low insertion loss filters. A drop in Q-value will result in a fast increase of insertion loss [1]. The most conventional way is to create these resonators by LC-tanks and couple them to each other using an inductor or a capacitor. The main goal in our design exercise was to reduce the area requirements as much as possible. We did this by omitting this coupling component and used the mutual inductance of the closely spaced tank inductors as coupling factor. In this manner we took advantage of the otherwise unwanted effect of inductor coupling and gained in area both by omitting the coupling component and by closely spacing the inductors.