Multi-layered Ultra-wideband (UWB) Bandpass Filters

In this paper, multi-layered ultra-wideband (UWB) bandpass filters have been proposed and developed. The first filter consists of two-layer dielectric substrate and one metal layer, modified from our original UWB bandpass filter using a single-layer microstrip-CPW (coplanar waveguide) structure. In this two-layered structure, we discovered from our simulation and experiments that some configuration of the filter can produce transmission zeros at the low and high sides of the passband which improve the skirt performance of the filter. The transmission zero was considered due to the direct coupling between the input and output ports. To miniaturize the filter length, a folded CPW, short-circuited at the right end, was introduced to reduce the length of the CPW. Measured results demonstrated the excellent filter performance: its frequency bandwidth form 3.1 GHz to 10.1 GHz, good insertion loss of 0.5 dB at the center frequency of passband, respectively. Parameter study of length dependence of the filter has been carried out based on EM simulator and experiments. The ratio of the filter length to the guided wavelength of the second higher transmission zero frequency keeps almost constant as 0.48. Finally, a four-layered filter was designed and the simulation results demonstrated that with suitable design, increasing the number of layers, as a multi-stage technique, can improve the skirt and out-band performance while keeps no size increase in length. A miniaturized filter was achieved with size about 10 mm times 5 mm times 1 mm, which is a fifth of the original single-layered filter.