Proceedings of Asia-Pacific Microwave Conference 2006
A Microstrip Bandpass Filter Using a Line Periodically Loaded with Unbalanced SIRs for Size Reduction and Spurious Suppression Jaruek Jantree* ,Thammarat Majaeng#, Sarawuth Chaimool#, and Prayoot Akkaraekthalin# *
Department of Electronics, Rajamangala University of Technology Suvarnabhumi, Suphanburi Campus 450 Moo 6,Yanyao District, Samchuk, Suphanburi 72130, Thailand.
#
Department of Electrical Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology North Bangkok, 1518 Pibulsongkram Rd., Bangsue, Bangkok 10800, Thailand Phone: +662-9132500 Ext.8519, Fax: +662-5856149 E-mail:
[email protected],
[email protected]
Abstract — A bandpass filter using a microstrip line periodically loaded with new unbalanced steppedimpedance resonators (SIRs) is proposed. With the proposed resonators, size reduction of approximately 16 % when comparing with the conventional SIR has been obtained. The bandpass filter has been designed at the operating frequency of 2 GHz with a narrow bandwidth of 60 MHz. The filter provides a lower insertion loss than that of parallel or cross-coupled structure with conventional SIRs. The upper stopband performance of the proposed filter has been drastically improved that the first spurious suppression of better than 20 dB has been measured. Index Terms — microstrip bandpass filter, steppedimpedance resonator (SIR), size reduction, spurious response suppression.
I. INTRODUCTION Microstrip bandpass filters are finding wide range of applications in wireless communication systems. The filter characteristics of low insertion losses, high selectivity, narrow bandwidth and compact size are always required. In selecting resonator types, filter structures must also be considered carefully. Traditionally, parallel-coupled line filters and hairpin filters are widely used in microwave circuits [1]. In order to enhance filter performances, cross-coupled filters which can realize elliptic or quasi-elliptic responses, have been proposed in [2-4]. Many other proposed filters use stepped-impedance resonators (SIRs) to improve the upper stopband performances [5-6]. However, the proposed SIR filters with parallel or cross-coupled structures provide spurious frequency shifts, but their levels are still high. Slow-wave wideband bandpass filters using SIRs with low losses have been proposed, however, these filters size are still comparably large [7]. Nevertheless most planar
Copyright 2006 IEICE
bandpass filter implemented with half-wave length resonators have inherently harmonic spurious frequencies. These spurious responses are essentially needed to be rejected, therefore, a cascaded lowpass or bandstop filter has been used to suppress them [8], but size and insertion loss are necessarily increased. In this paper, we propose a new bandpass filter using a microstrip line periodically loaded with unbalanced stepped-impedance hairpin resonators. The proposed filter topology significantly reduces insertion loss caused in parallel-coupled and crosscoupled structures by eliminating coupling gaps between resonators. Furthermore, this new filter structure is smaller comparing with filters having the same structures with conventional SIRs. The spurious responses of the filter have been also suppressed, resulting in the improved stopband characteristics. II. UNBALANCED STEPPED-IMPEDANCE RESONATOR Fig. 1 shows the proposed basic structure of an unbalanced stepped-impedance resonator that consists of two segments of transmission lines with different characteristic impedances of Z1 and Z2, corresponding to electrical lengths of T1 and T 2 . The resonator condition can be analyzed by deriving the input admittance Yi seeing from an open-end [9].
Yi
jY2
tan T 2 RZ tan T1 1 RZ tan T1 tan T 2
(1)
The resonance condition from Yi = 0 can be described as follows:
tan T 2 RZ tan T1
0
(2)
where RZ is the impedance ratio defined to be
RZ
.
Y1 / Y2
T1 , Z1
Ln
Z 2 / Z1
T2,Z2
Fig.1 An unbalanced stepped-impedance resonator structure. La1
Wa1
La3
Ga1
La2 La4
(a)
Wa2
Lb1
Wb1
Lb3
Gb1
Lb2 Wb2 Lb4
(b) Fig.2 (a) A conventional hairpin resonator and (b) the proposed hairpin resonator. 0
Magnitude (dB)
-10
§ § ·· ¨ T 2 tan 1 ¨ tan T 2 ¸ ¸ / S ¨ ¨ RZ ¸¹ ¸¹ © ©
(3)
When RZ = 1, the normalized resonator length Ln is a constant of one corresponding to a UIR. For RZ > 1, the normalized resonator length Ln becomes smaller than one. In this case, the resonator length is reduced compared to the UIR length and the size minimization can be achieved. Fig.2 (a) and (b) show structures of the conventional and the proposed unbalanced steppedimpedance hairpin resonators, respectively. The resonance responses of these resonators are evaluated by using the full-wave simulator, IE3D [10]. The resonators are designed on GML 1000 substrate, which has a given relative dielectric of 3.2, thickness of 30 mil and loss tangent of 0.004. The unbalanced SIR has been optimized resulting in the impedance ratio of RZ = 0.473. The sizes of the proposed resonator include Ga1 = Gb2 = 20 mil, Wa1 = Wb1 = 120 mil, Wa2 = Wb2 = 35 mil, La1 = Lb1 =514 mil, La2 = Lb2 = 260 mil, La3 = Lb3 = 105 mil, La4 = 85 mil, and Lb4 = 50 mil. The simulated results of resonance responses are given in Fig.3. These resonance responses are obtained from both structures with the same sizes. It can be noticed that the proposed resonator has a resonance frequency of 2.035 GHz, lower than 2.277 GHz of the conventional one. This means the unbalanced SIR can be made smaller than the conventional structure when they resonate at the same frequency. To compare their sizes at the same resonance frequency, the conventional resonator has the size of 613 x 260 mil2 while the size of unbalanced SIR is 514 x 260 mil2. We can see that, the size of the proposed unbalanced SIR is 16.15% smaller than the conventional one.
-20
III. FILTER IMPLEMENTATION AND RESULTS
-30 -40 -50 1.0
proposed (s11) conventional (s11) 1.5
2.0 Frequency (GHz)
2.5
3.0
Fig.3 S11 of the conventional resonator compared with the proposed resonator. The length Ln is the normalized resonator length, which is the ratio of the unbalanced SIR length and uniform impedance resonator (UIR) length, therefore, from (2) it can be written as
To demonstrate the usefulness of the proposed resonator, we have designed bandpass filters using a microstrip line periodically loaded with unbalanced SIRs. The first proposed filter has been designed as a structure shown in Fig.4. A substrate of GML 1000 with relative dielectric of 3.2, thickness of 30 mil and loss tangent of 0.004 has been utilized. The full-wave simulator IE3D has been used to finally determine the characteristics of the filter at a center frequency of 2 GHz and a narrow bandwidth of 60 MHz (FBW = 3%). The sizes of the designed
L1
L3
L1
L6
W1
L4
L3
L2
W1
W2
L5 L4
L2
Fig.8 Layout of the second filter. Fig.4 Layout of the first filter.
Fig.9 Photograph of the second filter.
Fig.5 Photograph of the first filter. 0
0 S11
-10 -30
S11
-10 S21
Magnitude (dB)
Magnitude (dB)
-20 -40 -50 -60 -70 -80
simulation measurement
-90 1.5
2.0 Frequency (GHz)
2.5
3.0
0 S11
Magnitude (dB)
S21
-30 -40 -50 -60
simulation measurement
-70 -80 1
S21
-40 -50 simulation measurement
-60
Fig.6 Simulated and measured S-parameters of the first filter.
-20
-30
-70
1.0
-10
-20
2
3 Frequency (GHz)
4
5
Fig.7 Simulated and measured S-parameters of the first filter at a wide frequency range.
1
2
3
4 5 6 Frequency (GHz)
7
8
9
Fig. 10 Simulated and measured S-parameters of the second filter at a wide frequency range. unbalanced SIR have been previously obtained from the last section. The other key dimensions of the proposed filter with a microstrip line periodically loaded with four unbalanced SIRs as shown in Fig.4 include W1 = 71.93 mil, L1 = 445 mil, L2 = 780 mil, L3 = 386 mil and L4 = 110 mil. A photograph of the fabricated filter is shown in Fig.5. Fig.6 shows comparisons between measured and simulated performances of the filter. We can see that the passband insertion loss is approximately 3 dB at the center frequency of 2.05 GHz, which is mainly due to the conductor loss of copper. The return loss is greater than 10 dB within passband. The two attenuation poles exhibit high rejection levels which are 73.5 dB at 1.81 GHz and 63.1 dB at 2.37 GHz,
respectively. However, the first spurious response at about 3.34 GHz is still very high as can be clearly seen in Fig.7. Obviously, with this filter structure, it can be concluded that the sizes can be reduced but the spurious responses can not be suppressed, therefore, the microstrip line has been modified to have stepped-impedances and a new arrangement of the proposed resonators has been performed, as a result of the second filter structure shown in Fig.8. The same specifications with the first implemented filter have been designed. With a substrate of GML1000, the resonator sizes are the same with the previous design and other dimensions of the second filter has been optimized using IE3D program, resulting in W1 = 71.93 mil, W2 = 20 mil, L1 = 240 mil, L2 = 360 mil, L3 = 473 mil, L4 = 185 mil, L5 = 286 mil, and L6 = 374.5 mil. The constructed filter is displayed in Fig.9. Fig.10 shows a wide frequency range of simulated and measured performances of this proposed filter, as we can notice that the spurious suppression is superior when compared with the first designed filter or the conventional filter. This filter exhibits a wide upper stopband with a rejection better than 17 dB up to about 7.6 GHz. This superior spurious suppression is caused by the stepped-impedance microstrip line and the proposed unbalanced SIRs properly loaded. IV. CONCLUSION A new bandpass filter using a microstrip line periodically loaded with the proposed unbalanced stepped-impedance hairpin resonators has been demonstrated. The size of the proposed resonators has been reduced for ~ 16%, when compared with the conventional SIR structure. Furthermore, the proposed filter has a lower insertion loss than the filters using parallel or cross-coupled steppedimpedance hairpin resonators of conventional structures. The filter not only has a compact size of resonators, but also provides an improved upper stopband characteristic. The measured filter responses agree very well with simulation expectations. The filter structure may be applied for several communication systems, when the superior spurious suppression is necessarily required. ACKNOWLEDGEMENT This work has been supported by the National Electronics and Computer Technology Center (NECTEC), Thailand, under the Project No. NT-B22-T2-38-47-13 and the Research No. 13/2547.
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