^{1}

^{*}

^{1}

^{1}

This paper presents a planar ultra-wideband (UWB) bandpass filter with sharp out-of-band rejection performance. The filter is formed by a folded multiple-mode resonator to realize high performance in an operation band from 3.3 to 10 GHz with a very compact size of 20 mm × 20 mm × 0.5 mm. An extra notched band centered at 5.8 GHz is further accomplished by etching a Hilbert fractal curve slit on the filter without the necessity of readjusting the geometrical parameters. The simulated and measured results are in good agreement.

Ultra-Wideband (UWB) systems have attracted increasing attention since the Federal Communications Commission released the unlicensed use of the frequency spectrum 3.1 - 10.6 GHz for UWB applications in 2002. With the rapid development of electronic products, high performance and compact size have been import issues in design considerations. The investigation on an UWB bandpass filter, which is one of the main components of UWB systems, has been a subject of interests.

Since 2005, various UWB bandpass filters have been designed and reported, including filters of composite lowpass and highpass structures [

Because of the possible interference with the existing wireless local area network or other applications, the research on UWB bandpass filters with notched bands has also been conducted in recent years. For example, etching slots on the patch or on the ground plane [

Based on [

The structure of the paper is as follows. In Section 2, the configurations of the proposed filters with or without the notched band are introduced and their resonance characteristics are analyzed. In Section 3, the experiment results of the above UWB filters are presented and compared with the simulated ones. Finally, conclusions are given in Section 4.

The configuration of the proposed UWB bandpass filter without a notched band is shown in _{i} and lengths of L_{i} (i = 1, 2, 3, 4). It is fed by a microstrip line with width W and parallel-coupled structure with width of W_{s} and lengths of L_{s}. This MMR structure is used to create five resonance frequencies in the passband to achieve UWB operation bandwidth by adjusting the parameters of the MMR. In addition, the stepped-impedance stub with widths of W_{3} and W_{4} and lengths of L_{3} and L_{4} can also introduce two transmission zeros, sharp rejection can then be realized by changing the locations of these transmission zeros to the edges of the passband.

_{i} and θ_{i} (i = 1, 2, 3, 4) are the characteristic admittances and electronic lengths, respectively. Since the MMR is symmetrical in structure, the even- and odd-mode methods can be used for its characterization.

where

From the resonance condition: _{e} or that of the odd-mode f_{o} can be obtained from

or

where_{z} can be produced by_{ }

Three _{z} can be moved to the edges of the passband by adjusting k_{3}, L_{3}, and L_{4}. The resonance frequencies f_{o} can be changed by adjusting k_{1}, L_{1}, and L_{2} and those of f_{e} by k_{1}, k_{2}, and k_{3}, and L_{i} (i = 1, 2, 3, 4). _{1}, k_{2}, and k_{3}. It is seen that f_{e} and f_{z} can be very close at both lower and upper frequencies, such that sharp rejection can be realized at the band edges.

The parameters W_{s} and S_{1} determine the coupling of the MMR and parallel-coupled structures.

Since a Hilbert fractal curve slit can produce a narrow notched band without increasing circuit area [_{h}. By fine-tuning the dimensions of the Hilbert fractal curve slit, the

notched band can be obtained in the desired frequency range.

The proposed filters with or without a narrow notched band are fabricated, as shown in _{1} = 0.2, W_{2} = 1.925, W_{3} = 2.25, W_{4} = 6, W_{s} = 0.18, L_{s} = L_{1} = 7.8, L_{2} = 6.75, L_{3} = 6.1, L_{4} = 7, S_{1} = 0.1, and S_{2} = 0.2.

_{h} = 4.4, y_{h} = 5, w_{h} = 0.3, and s_{h} = 2. It has a measured notched band

from 5.7 GHz to 6.0 GHz and the attenuation is less than −15 dB at the center frequency. The deviations of the measurements from the simulations may be attributed to tolerance in the fabrication process and diversity of material parameters.

This work was supported by the National High-Tech R & D Program (863 Program) under Grant 2011AA010201 and the Key Technologies R & D Program under Grant 2015ZX03002002..

Xueying Guo,Yunsheng Xu,Weidong Wang, (2015) Miniaturized Planar Ultra-Wideband Bandpass Filter with Notched Band. Journal of Computer and Communications,03,100-105. doi: 10.4236/jcc.2015.33017