PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012
588
Stepped Impedance Key-shaped Resonator for Bandpass and
Bandstop Filters Design
K. C. Lek and K. M. Lum
School of Electrical Engineering and Computer Science
University of Newcastle, Callaghan, NSW 2308, Australia
Abstract— In this paper, a bandpass filter (BPF) and bandstop filter (BSF) designs using Keyshaped resonators are presented. The filter designs comprise stepped impedance resonators (SIR)
and defective ground structure (DGS). The presence of the DGS creates a coupling capacitor in
the ground plane which provides a filtering channel for improving the insertion loss and bandwidth
performances. In addition, the DGS leads to reduction of overall circuit size, suppression of
harmonic response and high selectivity of the filter response. Conversely, the SIR resonators
are laid on the top conductor layer and the structure topology can be varied to obtain the
desired performance of the filter. Furthermore, the stopband characteristics can be controlled
by arranging the attenuation poles of the passband response. The proposed BPF is fabricated
on FR4 substrate having a thickness of 1.6 mm, loss tangent of 0.027 and relative permittivity
of 4.7. The best measured S11 response was observed at 2.6 GHz with a value of −20 dB. The
corresponding S21 is −4.2 dB, with a stopband attenuation of 21.8 dB/GHz. As an extension of
the SIR concept, a BSF is further proposed which consists of an open-ended stepped impedance
quarter-wavelength stub. The best measured S11 response was obtained 3.22 GHz with a value
of −0.2 dB. The stopband attenuation is 19.4 dB/GHz and the corresponding S21 is −28 dB. The
BSF is prototyped using FR4 substrate. The dimensions of both BPF and BSF are 23.5 mm
by 25.2 mm and 25.5 mm by 22.5 mm respectively. Validation of the BPF design is obtained via
good agreement between the simulated and measured results.
1. INTRODUCTION
Microwave filters are essential circuits in the wireless communication systems such as wireless LAN
and cellular telephone. To fulfil the current technology needs, a compact and high performance
filter had to be designed. Stepped Impedance Resonator (SIR) filter has been widely use because
of its low cost, light weight and simple structure [1–3].
In addition, defective ground structure (DGS) is introduced. DGS leads to reduction of overall
circuit size, suppression of harmonic response and high selectivity of the filter response [4, 5]. In this
paper, a BPF and BSF operating in the S-band are proposed using stepped impedance resonators
and DGS.
2. BANDPASS FILTER DESIGN AND CONCEPT
Design specifications of the proposed BPF are highlighted in Table 1. Fig. 1 depicts the BPF design
which comprises a single layer of FR4 substrate having a thickness of 1.6 mm. The top resonators
Figure 1: Configuration of proposed bandpass filter.
Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 589
1 and 2 are laid on the FR4 substrate. The structure of the resonators can be varied to obtain
the desired performance of the filter. Furthermore, the stopband characteristics of the BPF can be
controlled by arranging the attenuation poles of the passband response [6, 7].
3. BANDPASS FILTER SIMULATION AND MEASUREMENT RESULT
Figure 2 detailed the dimensional data of the proposed BPF. Advanced Design System (ADS)
simulation software was used to simulate the BPF design. Fig. 3 shows the simulated S11 and S21
results of the proposed BPF. The best match for the BPF is obtained at 2.62 GHz with a S11 value
of less than −25 dB. In the passband response, the corresponding S21 is approximately −3 dB with
an attenuation of 26.5 dB/GHz. The proposed BPF was fabricated on FR4 substrate as shown in
Fig. 4. Measurements were carried out using Network Analyzer.
As presented in Fig. 5, the best measured S11 was observed at 2.6 GHz with a value of −20 dB.
The corresponding S21 is −4.2 dB, with a stopband attenuation of 21.8 dB/GHz. The measured
−5 dB bandwidth over the passband is about 200 MHz. It is evident that the measured result agrees
well with the simulation results.
Table 1: Design specifications of bandpass filter.
Key Parameter
Operating Frequency (GHz)
Return Loss, S11 (dB)
Passband Insertion loss, S21 (dB)
Passband (MHz) at −5 dB
Stopband Attenuation
Value
2.6
< −10
> −10
200
> 20 dB/GHz
(a)
(b)
Figure 2: Key dimensional data of (a) top resonator and (b) defected ground plane.
(a)
Figure 3: Simulated S11 and S21 response of bandpass filter.
(b)
Figure 4: Fabricated bandpass filter. (a) Top resonators. (b) Ground plane.
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PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012
Figure 5: Simulated and measured S11 and S21 response of bandpass filter.
Table 2: Design specifications of bandstop filter.
Key Parameter
Operating Frequency
Return Loss
Stopband Insertion Loss, S21
Stopband (MHz) at −10 dB
Stopband Attenuation
Value
3.2 GHz
> −1 dB
< −10 dB
2000
> 10 dB/GHz
Figure 6: Key dimensional data of top resonator.
4. BANDSTOP FILTER DESIGN AND CONCEPT
As an extension of the SIR concept, a BSF is proposed. Design specifications of the proposed BPF
are highlighted in Table 2. As shown in Fig. 6, the BSF design consists of an open-ended stepped
impedance quarter-wavelength stub which comprises two characteristic impedances (Z1 , Z2 ) and
electrical lengths (Q1 , Q2 ) that leads to the resonating of two different frequencies (wf and wc ).
As illustrated in Fig. 7, the components encircled indicate the resonate circuits of the filter at
frequencies wf and wc . At the resonant frequencies, wf and wc , the circuits are able to exhibit a
stopband response that rejects unwanted signal thus enabling the filter to operate as a BSF [8–10].
5. BANDSTOP SIMULATION AND MEASUREMENT RESULT
Figure 8 shows the simulated S11 and S21 response of the proposed BSF. The best return loss,
S11 for the BSF is obtained at 3.28 GHz with a corresponding S11 value of −0.1 dB. The stopband
attenuation is 17.4 dB/GHz. In the stopband response, S21 is approximately −38 dB.
The proposed BSF was prototyped using one FR4 substrate with the SMA connectors soldered
onto the microstrip feed line as shown in Fig. 9. Measurements were carried out using Network
Analyzer.
As presented in Fig. 10, the best measured S11 was obtained at 3.22 GHz with a value of
Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 591
(a)
(b)
Figure 7: Equivalent circuit of (a) configuration of stepped impedance resonator, (b) bandstop filter.
(a)
Figure 8: Simulated S11 and S21 response of bandstop filter.
(b)
Figure 9: Fabricated bandstop filter. (a) Top resonator. (b) Ground plane.
Figure 10: Simulated and measured S11 and S21 response of bandstop filter.
−0.2 dB. The stopband attenuation is 19.4 dB/GHz and the corresponding S21 is −28 dB. The
measured −10 dB bandwidth over the stopband is about 1800 MHz. Despite of the slight variance
in the results due to unexpected fabrication tolerance, material losses and parasitic effect of the
SMA connectors, it is evident that the measured result agrees well with the simulation results.
6. CONCLUSION
The design of a bandpass filter based on the configuration of stepped impedance key-shaped resonators with defected ground structure has been validated through simulation and practical measurement. It has also been demonstrated that the stepped impedance key-shaped resonator topology
of the proposed bandpass filter design can be further modified to obtain a bandstop filter response.
The proposed filters have the advantages of providing reasonably acceptable filtering function using simple resonator and defected ground structure topology and are easy to be fabricated. The
proposed configuration seems particularly suitable for use in multilayer transceiver designs [11–14]
and localization applications [15–18].
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PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012
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