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 [1], shorted-circuited stub filters [2] [3], multiple-mode resonator (MMR) structure filters [4], etc. Modification or improvement of MMR filters has also been proposed due to their compact size and high performance [5]-[9]. In [7] [8], the transmission zeros of MMR filters are used to realize sharp out-of-band rejection performance.

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 [10] [11], using asymmetric coupled lines [12], and adding notch resonators [13] [14].

Based on [7], a miniaturized UWB filter formed by a MMR is realized in this paper. The MMR is formed by a stepped-impedance resonator with a stepped-impedance stub. Different from [7], however, the UWB bandpass filter here achieves UWB with a much more compact size of 20 mm × 20 mm by folding the MMR. An extra notched band around 5.8 GHz is further obtained by etching a Hilbert fractal curve slit [15] on the filter without the necessity of readjusting the geometrical parameters. The filter is fabricated on a printed circuit board with a relative permittivity of 2.65 and thickness of 0.5 mm.

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 proposed filters with or without a narrow notched band are fabricated, as shown in Figure 7, and measured to demonstrate their performance. A SMA connector is attached to the 50 ohm microstrip feeding line of W = 1.35 mm. SMA connectors are included in the simulation model. The overall sizes of the filters are 20 mm × 20 mm. The simulation shows that the dimensions of the metallic boxes have little influence on the performance. The structure parameters are: W₁ = 0.2, W₂ = 1.925, W₃ = 2.25, W₄ = 6, W_s = 0.18, L_s = L₁ = 7.8, L₂ = 6.75, L₃ = 6.1, L₄ = 7, S₁ = 0.1, and S₂ = 0.2.

Figure 8 illustrates the simulated and measured results of the UWB bandpass filter without a notched band. The measured pass band is from 3.3 GHz to 10 GHz with a return loss less than −10 dB.

Figure 9 depicts the simulated and measured results of the UWB bandpass filter with a narrow notched band. The design parameters for the notch are: x_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.

Figure 6. Simulated and measured S parameters of the proposed UWB filter with a notched band.

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