2008 Volume 5 Issue 8 Pages 291-295
In this paper, a two-layered ultra-wideband (UWB) bandpass filter has been proposed and developed. The filter consists of two-layered dielectric substrate and one metal layer, modified from our original UWB bandpass filter using a single-layer microstrip-CPW (coplanar waveguide) structure. To miniaturize the filter length, a folded CPW, short-circuited at the right end, was introduced to reduce the length of the CPW. 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. Measured results demonstrated the excellent filter performance: its frequency bandwidth form 3.1GHz to 10.1GHz, good insertion loss of 0.5dB at the center frequency of passband, respectively. Parameter study of length dependence of the filter has been carried out based on EM simulator. The ratio of the filter length to the guided wavelength of the second higher transmission zero frequency keeps almost constant as 0.48.