Power divider with microstrip electromagnetic bandgap element for miniaturisation and harmonic...
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Power divider with microstrip electromagnetic bandgap element for miniaturisation and harmonic rejection F. Zhang and C.F. Li A novel technique for harmonic suppression and size reduction in planar Wilkinson power divider is presented. The proposed technique served by a microstrip electromagnetic bandgap element (MEBE) is used to reject the unwanted harmonics by employing its stopband effect and to reduce the length of microstrip by utilising its slow- wave effect. The whole size of the proposed divider has been reduced by 39% compared to the conventional power divider. Higher harmonics are effectively suppressed. Introduction: Miniaturisation and harmonic suppression are two extre- mely important requirements for modern wireless communication systems to reduce cost, facilitate portable use and improve communi- cation quality. To achieve this, tremendous efforts have been contributed to electromagnetic bandgap structures (EBS) such as defected ground structure (DGS). Two remarkable features of EBS are slow-wave and stopband effects, which can enable size reduction and unwanted fre- quency rejection, respectively. Compact microstrip resonant cell (CMRC), as a new wave of MEBE technology, attracts much attention owing to its easy implementation, free lumped elements and free ground suspension [1–6]. CMRC integrates stopband and slow-wave effects owing to increased equivalent semi-lumped distributed resonant inductance in series and shunt capacitance. Design of IBCMRC: In [6], we proposed beeline CMRC (BCMRC) and built a lowpass filter using three BCMRCs in series. Based on this, this Letter describes a compact Wilkinson power divider using improved BCMRC (IBCMRC) resonators for harmonic suppression and size reduction. Design motivation is to enable IBCMRC stronger slow- wave effects and a better impedance match, together with tunable stopbands. The BCMRC is available if the patterns in the virtual rectangle (two compensated stub lines) are removed from IBCMRC, as shown in Fig. 1. In this Letter, all devices are designed with 1.8 GHz operating frequency on substrate with relative dielectric constant 1 r ¼ 2.65 and thickness h ¼ 1 mm. Here, the designed IBCMRC line is the 70.7 V impedance line (1.53 mm wide) for the power divider. Fig. 1 Layout of IBCMRC structure (L 1 ¼ 0.3 mm, L 2 ¼ 9.4 mm, L 3 ¼ 10 mm, D 1 ¼ 0.3 mm, D 2 ¼ 0.8 mm, D 3 ¼ 1.1 mm, D 4 ¼ 1.5 mm, l ¼ 18.5 mm, L ¼ 29 mm) Fig. 2 Simulated S-parameters of BCMRC and IBCMRC Fig. 2 shows simulated S-parameters of BCMRC and IBCMRC. It is worth noting that there are two deep stopbands at 3.6 and 5.4 GHz, which correspond to the 2nd and 3rd harmonics of the operating fre- quency, respectively. Thus when the IBCMRC is introduced in the power divider, the two deep inherent stopbands can easily suppress unwanted harmonic signals with elimination of an additional separate filter. Fig. 3 shows phase performance of the conventional microstrip, BCMRC and IBCMRC with the same physical length. It can obviously be seen that the IBCMRC has the strongest miniaturisation ability in the three types of microstrip transmission lines owing to the slow-wave effect of BCMRC and the equivalent shunt capacitance loaded by two compensated stub lines. Fig. 3 Phase performance with same physical length Fig. 4 Smith chart of S 11 a BCMRC b IBCMRC As shown in Fig. 4, the S 11 Smith charts of BCMRC and IBCMRC indicate that a high reflection coefficient of BCMRC will result in a high insertion loss. This can be understood as follows: the characteristic impedance of the BCMRC section at low frequency does not match properly. In the BCMRC, both series inductance and shunt capacitance are increased in order to achieve the desired slow-wave and stopband characteristics. However, full-wave analysis shows that the characteristic impedance of the BCMRC at the lowpass band is higher than 70.7 V. The reason is that the equivalent series inductance is increased more than the shunt capacitance. The port impedance escapes away from 70.7 V at lowpass frequency. Hence, currently it is not suitable for the 70.7 V impedance line design in power dividers. According to the theory of the artificial transmission line and capacitive loading effect, two meander stub lines are added to compensate for the unbalance. By tuning different length of L and l in Fig. 1, it is found that the longer line can be resonant at the 2nd harmonic signal of the fundamen- tal frequency. The shorter line can be resonant at the 3rd harmonic signal. Design of power divider: The widths, gaps and lengths of the IBCMRC were adjusted manually and optimised with a commercial EM simulator. It is found that conventional 0.25 l g microstrip with a length of 29 mm can be replaced by IBCMRC only with a length of 18.6 mm, which cor- respond to 0.16 l g at a centre frequency of 1.8 GHz. Hence the length is reduced by 36%. As for the whole size of the power divider, the effective areas are 2.34 and 1.44 cm 2 for conventional and proposed IBCMRC- based power dividers, respectively. This indicates that IBCMRC can prominently reduce the area of power divider by 39% to save die cost. The measured performance results are shown in Fig. 5. The BCMRC- based power divider exhibits an insertion loss not greater than 3.6 dB at the operating frequency, which is comparable to the one with conven- tional microstrip. The measured reflection coefficients are below 20 dB ELECTRONICS LETTERS 13th March 2008 Vol. 44 No. 6
Transcript of Power divider with microstrip electromagnetic bandgap element for miniaturisation and harmonic...
Power divider with microstripelectromagnetic bandgap element forminiaturisation and harmonic rejection
F. Zhang and C.F. Li
A novel technique for harmonic suppression and size reduction inplanar Wilkinson power divider is presented. The proposed techniqueserved by a microstrip electromagnetic bandgap element (MEBE) isused to reject the unwanted harmonics by employing its stopbandeffect and to reduce the length of microstrip by utilising its slow-wave effect. The whole size of the proposed divider has beenreduced by 39% compared to the conventional power divider. Higherharmonics are effectively suppressed.
Introduction: Miniaturisation and harmonic suppression are two extre-mely important requirements for modern wireless communicationsystems to reduce cost, facilitate portable use and improve communi-cation quality. To achieve this, tremendous efforts have been contributedto electromagnetic bandgap structures (EBS) such as defected groundstructure (DGS). Two remarkable features of EBS are slow-wave andstopband effects, which can enable size reduction and unwanted fre-quency rejection, respectively. Compact microstrip resonant cell(CMRC), as a new wave of MEBE technology, attracts much attentionowing to its easy implementation, free lumped elements and freeground suspension [1–6]. CMRC integrates stopband and slow-waveeffects owing to increased equivalent semi-lumped distributed resonantinductance in series and shunt capacitance.
Design of IBCMRC: In [6], we proposed beeline CMRC (BCMRC) andbuilt a lowpass filter using three BCMRCs in series. Based on this, thisLetter describes a compact Wilkinson power divider using improvedBCMRC (IBCMRC) resonators for harmonic suppression and sizereduction. Design motivation is to enable IBCMRC stronger slow-wave effects and a better impedance match, together with tunablestopbands.
The BCMRC is available if the patterns in the virtual rectangle (twocompensated stub lines) are removed from IBCMRC, as shown in Fig. 1.In this Letter, all devices are designed with 1.8 GHz operating frequencyon substrate with relative dielectric constant 1r ¼ 2.65 and thickness h ¼
1 mm. Here, the designed IBCMRC line is the 70.7 V impedance line(1.53 mm wide) for the power divider.
Fig. 1 Layout of IBCMRC structure
(L1 ¼ 0.3 mm, L2 ¼ 9.4 mm, L3 ¼ 10 mm, D1 ¼ 0.3 mm, D2 ¼ 0.8 mm, D3 ¼1.1 mm, D4 ¼ 1.5 mm, l ¼ 18.5 mm, L ¼ 29 mm)
Fig. 2 Simulated S-parameters of BCMRC and IBCMRC
Fig. 2 shows simulated S-parameters of BCMRC and IBCMRC. It isworth noting that there are two deep stopbands at 3.6 and 5.4 GHz,which correspond to the 2nd and 3rd harmonics of the operating fre-quency, respectively. Thus when the IBCMRC is introduced in thepower divider, the two deep inherent stopbands can easily suppress
ELECTRONICS LETTERS 13th March 2008 Vol. 44
unwanted harmonic signals with elimination of an additional separatefilter. Fig. 3 shows phase performance of the conventional microstrip,BCMRC and IBCMRC with the same physical length. It can obviouslybe seen that the IBCMRC has the strongest miniaturisation ability in thethree types of microstrip transmission lines owing to the slow-waveeffect of BCMRC and the equivalent shunt capacitance loaded by twocompensated stub lines.
Fig. 3 Phase performance with same physical length
Fig. 4 Smith chart of S11a BCMRCb IBCMRC
As shown in Fig. 4, the S11 Smith charts of BCMRC and IBCMRCindicate that a high reflection coefficient of BCMRC will result in ahigh insertion loss. This can be understood as follows: the characteristicimpedance of the BCMRC section at low frequency does not matchproperly. In the BCMRC, both series inductance and shunt capacitanceare increased in order to achieve the desired slow-wave and stopbandcharacteristics. However, full-wave analysis shows that the characteristicimpedance of the BCMRC at the lowpass band is higher than 70.7 V.The reason is that the equivalent series inductance is increased morethan the shunt capacitance. The port impedance escapes away from70.7 V at lowpass frequency. Hence, currently it is not suitable for the70.7 V impedance line design in power dividers. According to thetheory of the artificial transmission line and capacitive loading effect,two meander stub lines are added to compensate for the unbalance.By tuning different length of L and l in Fig. 1, it is found that thelonger line can be resonant at the 2nd harmonic signal of the fundamen-tal frequency. The shorter line can be resonant at the 3rd harmonicsignal.
Design of power divider: The widths, gaps and lengths of the IBCMRCwere adjusted manually and optimised with a commercial EM simulator.It is found that conventional 0.25 lg microstrip with a length of 29 mmcan be replaced by IBCMRC only with a length of 18.6 mm, which cor-respond to 0.16 lg at a centre frequency of 1.8 GHz. Hence the length isreduced by 36%. As for the whole size of the power divider, the effectiveareas are 2.34 and 1.44 cm2 for conventional and proposed IBCMRC-based power dividers, respectively. This indicates that IBCMRC canprominently reduce the area of power divider by 39% to save die cost.The measured performance results are shown in Fig. 5. The BCMRC-based power divider exhibits an insertion loss not greater than 3.6 dBat the operating frequency, which is comparable to the one with conven-tional microstrip. The measured reflection coefficients are below 20 dB
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at three ports. The measured isolation between ports 2 and 3 is betterthan 20 dB at the operating frequency. For the conventional Wilkinsonpower divider, higher harmonics exist periodically, which are unwantedin communication systems. The proposed IBCMRC power divider oper-ates in the same way as the conventional one, but effectively rejects the2nd and 3rd spurious harmonics by above 226 and 225 dB, respect-ively. Table 1 gives a comparison between the conventional and pro-posed power dividers.
Fig. 5 Measured S-parameters for IBCMRC-based power divider
Table 1: Comparisons of conventional and proposed Wilkinsonpower dividers
Power dividerCircuit area
(cm2)
Relativearea(%)
Harmonicsuppression
(dB)
2nd 3rd
Conventional 2.34 100 no no
Proposed 1.44 61 226 225
Conclusions: This Letter has described a novel microstrip electromag-netic bandgap element (IBCMRC) and its application in power dividers
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for harmonic suppression and miniaturisation. The developed powerdivider rejected the 2nd and 3rd harmonic signals by 225 dB whilereducing circuit area by 39% without sacrificing the characteristics. Inminiaturisation and harmonic rejection of various microwave circuits,the proposed IBCMRC technique may have wide applications. The pro-posed power divider was fabricated with a standard PCB processwithout rigorous and additional processes such as lumped components,ground etch, multilayer metal, via holes or bonding wire.
# The Institution of Engineering and Technology 200824 December 2007Electronics Letters online no: 20083693doi: 10.1049/el:20083693
F. Zhang and C.F. Li (Department of Physics, Wuhan University,Wuhan 430072, People’s Republic of China)
E-mail: [email protected]
References
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