Miniaturized Planar Ultra-Wideband Bandpass Filter with Notched Band ()
1. Introduction
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.
2. Filter Structure and Design
2.1. Configuration and Design
The configuration of the proposed UWB bandpass filter without a notched band is shown in Figure 1, which consists of a folded MMR formed by a stepped-impedance resonator with widths of Wi and lengths of Li (i = 1, 2, 3, 4). It is fed by a microstrip line with width W and parallel-coupled structure with width of Ws and lengths of Ls. 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 W3 and W4 and lengths of L3 and L4 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.
Figure 2(a) shows the MMR structure. Yi and θi (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. Figure 2(b) and Figure 2(c) are its even- and odd-mode equivalent circuits, respectively. The symmetrical planes are open-circuited for even-mode excitation and short-circuited for odd- mode excitation. The input admittances 
and 
for the even- and odd-mode excitations can be respectively given bellow
, (1)
, (2)
where
, (3)
, (4)
![]()
Figure 1. Configuration of the proposed UWB filter without notched band.
From the resonance condition: 
or
, the resonance frequency of the even-mode fe or that of the odd-mode fo can be obtained from
, (5)
or
, (6)
where
, 
, and
. The transmission zeros fz can be produced by
.
Three ![]()
two![]()
, and two ![]()
can be determined within the passband. To realize sharp rejection, the locations of the transmission zeros fz can be moved to the edges of the passband by adjusting k3, L3, and L4. The resonance frequencies fo can be changed by adjusting k1, L1, and L2 and those of fe by k1, k2, and k3, and Li (i = 1, 2, 3, 4). Figure 3 plots the resonance frequencies and transmission zeros of the MMR with varying k1, k2, and k3. It is seen that fe and fz can be very close at both lower and upper frequencies, such that sharp rejection can be realized at the band edges.
The parameters Ws and S1 determine the coupling of the MMR and parallel-coupled structures. Figure 4 illustrates the ![]()
simulated by HFSS with weak and strong coupling. From the weak coupling the resonance frequencies and transmission zeros can be obviously observed. With strong coupling and the MMR characteristics, an UWB bandpass filter with high performance can be realized.
2.2. UWB Filter with Notched Band
Since a Hilbert fractal curve slit can produce a narrow notched band without increasing circuit area [15], it’s utilized and etched on the stepped-impedance stub to produce a notched band around 5.8 GHz, while the other structure parameters remain unchanged, as shown in Figure 5. Figure 6 depicts the change of resonant frequencies with different values of![]()
, ![]()
or wh. By fine-tuning the dimensions of the Hilbert fractal curve slit, the
![]()
![]()
(a) (b) (c)
Figure 2. (a) Configuration of MMR; (b) Even-mode equivalent circuit; (c) Odd-mode equivalent circuit.
![]()
![]()
![]()
(a) (b) (c)
Figure 3. Resonace characteristics of MMR. (a) With varying k3. k1 = 1, k2 = 0.2; (b) With varying k2. k1 = 1, k3 = 0.2; (c) With varying k1. k2 = 0.2, k3 = 0.2. L1 = 8 mm, L2 = 7 mm, L3 = 10 mm, and L4 = 4 mm.
![]()
Figure 4. Simulated ![]()
with weak and strong coupling.
![]()
Figure 5. Configuration of the filter with notched band.
![]()
Figure 6. Simulated ![]()
of the notched band.
notched band can be obtained in the desired frequency range.
3. Experiment Results and Discussions
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, Ws = 0.18, Ls = 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: xh = 4.4, yh = 5, wh = 0.3, and sh = 2. It has a measured notched band
![]()
(a) (b)
Figure 7. Photographs of the filters. (a) Without a notched band; (b) With a notched band.
![]()
Figure 8. Simulated and measured S parameters of the proposed UWB filter without 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.
4. Conclusion
Figure 6. Simulated and measured S parameters of the proposed UWB filter with a notched band.
Acknowledgements
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..