Recent progress in planar microwave filter technology

This paper aims to review recently developed planar microwave filter technologies, including the follows. Liquid crystal polymer is a new and promising thermoplastic material. The liquid crystal polymer film has excellent electrical characteristics such as stable low dielectric constant of 3.15 in a wide frequency range and low loss tangent of 0.002-0.004 at millimetre-wave frequency. It has extraordinary barrier properties comparable to that of glass and low coefficient of thermal expansion. Moreover, it also has low moisture absorption. These characteristics make liquid crystal polymer can be used as both substrate and package material. Two types of liquid crystal polymer films such as bonding film and core film are available with different melting temperatures. In fabrication, by controlling fabrication temperature, the bonding film can bond core films together. Thus, it is possible for liquid crystal polymer film to implement multilayer architectures. The cost of liquid crystal polymer (LCP) is comparable to that of conventional print-circuit-board material and is much cheaper than that of Low Temperature Co-fired Ceramic (LTCC). LCP can package both active and passive devices in compact, vertically integrated RF modules using homogeneous multilayer dielectric laminations at a low temperature (about 290 °C), which would be more challenging for the LTCC technology due to its much higher (~850 °C) processing temperature. The unique combination of properties makes liquid crystal polymer technology ideally suitable for designing compact microwave circuits and high density system-in-package applications. To this end, several planar ultra-wideband (UWB) filters based on multilayer LCP circuit technology will be presented with both simulated and measured results. Electronically reconfigurable or tunable mic