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Ultra-wideband balun and power divider using coplanar waveguide to microstrip transitions
Accepted Manuscript Ultra-Wideband Balun and Power Divider Using Coplanar Waveguide to Microstrip Transitions Ali K. Horestani, Zahra Shaterian PII: DOI: Reference:
S1434-8411(18)31853-3 https://doi.org/10.1016/j.aeue.2018.08.024 AEUE 52466
To appear in:
International Journal of Electronics and Communications
Received Date: Accepted Date:
11 July 2018 18 August 2018
Please cite this article as: A.K. Horestani, Z. Shaterian, Ultra-Wideband Balun and Power Divider Using Coplanar Waveguide to Microstrip Transitions, International Journal of Electronics and Communications (2018), doi: https:// doi.org/10.1016/j.aeue.2018.08.024
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Ultra-Wideband Balun and Power Divider Using Coplanar Waveguide to Microstrip Transitions Ali K. Horestania , Zahra Shaterianb a Aerospace
Research Institute, Ministry of Science, Research and Technology, Tehran 1465774111, Iran b School of Electrical and Computer Engineering, College of Engineering, University of Tehran, Tehran 143995713, Iran
Abstract An ultra-wideband transition from coplanar waveguide (CPW) to a pair of microstrip lines is presented in this letter. It is shown that the proposed transition can be used to devise an ultra-wideband T-junction power divider with equally split in-phase output signals. More importantly, it is demonstrated that the outof-phase magnetic currents in the slots of the CPW can be utilized to achieve an out-of-phase power divider or a balun. Both the T-junction power divider and the balun benefit from compact size and good performance over an ultra wide frequency band from 3 to 16 GHz. Prototypes of both proposed devices are fabricated and measured to provide an experimental verification on the concept and numerically predicted features. Keywords: Power divider, balun, transition, planar structures, ultra-wideband.
1. Introduction Significant attention has been given to ultra-wideband systems due to their high communication data rate, immunity to multipath interferences and low power consumption. As a result, enhancing the operating bandwidth of passive microwave devices while avoiding complex structures and maintaining the ∗ Corresponding
Author: Ali K. Horestani Email addresses: [email protected] (Ali K. Horestani), [email protected] (Zahra Shaterian)
Preprint submitted to Journal of LATEX Templates
August 13, 2018
device miniaturization has been subject of interest in academic and industrial researches in recent years [1]. With this aim, many researches have been carried out in the realization of ultra-wideband structures using coplanar waveguide (CPW) and coplanar strip (CPS) transmission lines (TLs) because these technologies are compact, low profile, easy for fabrication and as a result cost effective. These TLs also allow series and shunt mounting of lumped active and passive components. An important difference between the two types of TLs is that CPW is an unbalanced TL, whereas the CPS is a balanced line. Thus, to integrate different microwave circuits with these two planar structures, CPW-to-CPS transitions are required. For instance in many applications a symmetric (balanced) planar antenna, such as a bow-tie or dipole antenna, needs to be fed by a coaxial cable or CPW, which are unbalanced TLs. A balun that converts a balanced signal to an unbalanced one is commonly used in such cases [2]. Power dividers are other fundamental passive components, which are widely used in a variety of the microwave and millimeter-wave systems such as antenna array feeding networks, mixers and modulators [3, 4]. In contrast to the baluns, power dividers usually provide in-phase output signals. Due to the wide application of baluns and power dividers in microwave systems, various in-phase and out-of-phase CPW-to-CPS transitions have been reported in the literature. Baluns and power dividers in [5, 6, 7, 8, 9, 10, 11] that are based on half or quarter wavelength TLs are relatively large in size and narrow band. To achieve ultra-wideband CPW-to-CPS transitions, which are required for feeding wideband or multiband antennas, several structures have been reported in the literature. However, they require multi-layer substrates [12, 13], via holes [14], or air-bridges and bond wires [15]. Metamaterial-inspired baluns and power dividers such as those reported in [16, 17, 18, 19, 20], albeit compact, are narrow band, and usually require many vias and lumped or semilumped passive components. In order to avoid the complexities associated with the fabrication procedure of multi-layer structures and air-bridges, planar baluns and power dividers were presented in [21, 22, 23]. The reported structures 2
cover an ultra-wide band, however they are only appropriate for realization in microstrip technology. In this paper, new designs for ultra-wideband T-junction power divider and balun are presented. Section 2 is devoted to the design of an ultra-wideband Tjunction power divider. It is shown that, the required impedance transformation is achieved from the slotline-to-microstrip transitions rather than quarter wavelength transformers. As a result a compact power divider with ultra-wideband performance is achieved. It is also shown in Sec. 3 that the proposed in-phase power divider can be readily changed to a balun in which the required 180-degree phase difference between the two output ports is achieved through the relative polarity of magnetic currents in the slots of the CPW. Therefore, the proposed balun presents an ultra-wideband performance. In both cases, prototypes of the designed structures are fabricated and the simulation results are experimentally 50
verified. Section 4 concludes the paper.
2. Ultra-Wideband T-Junction Power Divider This section is devoted to the design of an ultra-wideband T-junction power divider. The proposed power divider is essentially developed based on a slotlineto-microstrip transition. Thus, to make the paper smoother the first subsection provides a brief review on the literature related to the slotline-to-microstrip transition. The section then presents the ultra-wideband T-junction power divider and validates the designed structure both numerically and experimentally. 2.1. Slotline-to-Microstrip Transition Fig. 1 (a) shows an illustration of the well-known structure of a slotline-tomicrostrip transition, which was experimentally investigated in [24]. As shown in the figure, the microstrip line (in yellow color) on one side of the substrate is crossed by a slotline, which is etched in the ground plane (in orange color) of the microstrip on the back side of the substrate. A comprehensive study on different methods to extend the achievable bandwidth and flatness of the
3
frequency response of the transition was conducted in [24]. It was shown that if the slotline is extended with a quarter wavelength shorted stubs, while the microstrip is shorted to the ground plane through a via-hole, the maximum bandwidth can be achieved. The study, however, showed that a passband with superior flatness can be achieved if the microstrip and slotline are extended with quarter wavelength open and short stubs, respectively. It was shown that the performance of the transition in terms of bandwidth and flatness can be further improved if the open/short stubs are realized in a radial shape, as shown in Fig. 1 (b). The equivalent circuit model for the slotline-to-microstrip transition with the excitation on the slotline side is depicted in Fig. 2. In this model, which was developed and validated by Sch¨ uppert in [24], the characteristic impedances of the slotline and microstrip are denoted by Zs and Zm , respectively. Since the characteristic impedances of the short and open stubs can generally be different from those of the slotline and microstrip, they are denoted by Zss and Zom . The electrical length of these stubs are shown by θss and θom . The effect of the discontinuities of the shorted slotline and open microstrip stub are modeled with Lss and Com , respectively. The transformer with turns ratio n in the equivalent circuit model represents the magnetic coupling between the two TLs. It is very important to note that the characteristic impedances of the slotline and the microstrip are generally different. Thus, the required impedance matching is conducted by the magnetic coupling between the two TLs, which is properly modeled by a transformer in the equivalent circuit model. 2.2. Ultra-Wideband T-Junction Power Divider By extending the concept of slotline-to-microstrip transition of the previous subsection, a transition from CPW to a pair of microstrip lines or in other words a T-junction power divider is developed in this section. An illustration of the proposed ultra-wideband T-junction power divider is shown in Fig. 3. In this illustration, the CPW, which is excited through a coaxial connector is on the top layer of the substrate, while a pair of microstrip 4
(a)
(b)
Figure 1: Microstrip to slotline transition with (a) uniform λ/4 extensions, (b) circular stub extensions. The microstrips are illustrated with yellow color and solid lines, while dotted lines are used to indicate the boundaries of the slotlines etched in the ground plane.
Figure 2: Circuit model for the slotline-to-microstrip transition.
5
lines are on the back side of the substrate. The CPW can be considered as a pair of slotlines. Due to the symmetry of the CPW structure the power guided by the CPW is already equally divided between its slotlines. Handwritten font is used in this figure and also in the text to represent instantaneous (rather than time harmonic) field vectors. Note that, as shown in the zoomed view of Fig. 3, 100
in the CPW mode the magnetic currents H1
M1
and
M2
and also magnetic fields
◦
and H2 are 180 out of phase. However, as shown in the figure, since each
of the slotline-to-microstrip transitions is the mirror image of the other one, the induced electric currents
J1
and
J2
flowing in the microstrip lines are in phase.
That is, the structure acts as an in-phase power divider. The equivalent circuit model of the proposed power divider, which is developed based on the equivalent circuit model of a slotline-to-microstrip transition (Fig. 2) is shown in Fig. 4. In order to validate the concept, an ultra-wideband T-junction power divider is designed and numerically simulated. Also, a prototype of the device is fabricated and measured. Rogers RT5880 material with thickness 0.51 mm, dielectric constant r = 2.2, and loss tangent of 0.0009 is used as the substrate. Dimensions of the proposed T-junction power divider are given in the caption of Fig. 3. Photographs of the top and bottom views of the fabricated prototype of the ultra-wideband T-junction power divider are shown in Fig. 5. Simulated and measured reflection coefficient at port 1 and transmission coefficients |S21 | and |S31 | of the power divider are depicted in Fig. 6. The solid lines show the simulation results, whereas the experimental results are shown by dashed lines. The figure shows that the simulated insertion loss is less than 1 dB across a wide frequency band from 3 to 16 GHz. The figure also shows that a simulated return loss better than 15 dB is achieved over the frequency band of 3 to 16 GHz. This wideband operation is due to the strong coupling between each slot of the CPW and the corresponding microstrip line, acting as a wideband impedance transformer. Note that in a conventional T-junction power divider, matching at the input port with a source impedance of 50 Ω is achieved when the two output transmission lines are 100 Ω lines. Thus, quarter6
Figure 3: Layout of the proposed ultra-wideband T-junction power divider. The dimensions of the structure are: wg = 0.2 mm, wc = 4.3 mm, and wm = 1.5 mm (corresponding to 50 Ω lines). Other dimensions are: s = 4.5 mm, d = 10 mm, and rm = rc = 2 mm.
Com θom , Zom
Zs Lss
θss , Zss n:1
Port 1
θom , Zom
Zs Lss
Zm
Port 2
Com
Zm
θss , Zss n:1
Figure 4: Circuit model for the T-junction power divider.
7
Port 3
Figure 5: Photographs of a fabricated prototype of the proposed ultra-wideband T-junction power divider (a) top view, and (b) bottom view.
wave transformers have to be used to bring the impedances of the output lines back to the desired levels [25]. The disadvantage of this method is that the operating bandwidth of the power divider will be limited to the bandwidth that the quarter-wave transformers can provide [25]. In contrast, in the proposed T-junction power divider 50 Ω TLs are used for all three ports of the structure. The reason is that in the proposed structure the impedance transformation is carried out by the coupling between the CPW and microstrip lines. This feature, which is modeled by a pair of transformers in the equivalent circuit model of Fig.4, bypasses the need for quarter-wave transformers, and results in an ultra-wideband performance. Figure 6 shows that the insertion loss at frequencies higher than 14 GHz is slightly higher than expected. This loss can be attributed to free-space radiation and surface wave losses, which are more pronounced at higher frequencies. The figure also shows that the measured insertion loss at frequencies higher than 14 GHz is slightly higher than the simulated one. This discrepancy most probably arises from the fabrication tolerances and imperfect SMA connectors at those high frequencies.
The simulated (solid lines) and measured (dashed lines)
reflection from output port (|S22 |) and the isolation between the output ports (|S23 |) are depicted in Fig. 7. The figure shows that return loss at the output ports is better than 6 dB, while an isolation around 10 dB is achieved between the output ports. Note that this level of isolation is typical of a T-junction power
8
0
S−parameter (dB)
−5 −10 −15 −20 −25
S11
−30
S21
−35
S31
−40
0
5
10 Frequency (GHz)
15
20
Figure 6: Simulated (solid lines) and measured (dashed lines) input reflection coefficient (|S11 |) and transmission coefficients (|S21 | and |S31 |) of the proposed ultra-wideband T-junction power divider.
dividers, which are lossless three-port structures [25]. Increasing the isolation and return loss at the output ports by introducing lossy components to form a Wilkinson power divider can be the subject of a future study. It is also im150
portant to note that the structure is completely symmetric. Thus, theoretically the output signals must be identical both in magnitude and phase. Therefore, any phase difference in numerical or experimental results has to be assigned to numerical errors and imperfect fabrication of the structure. Measured group delay of the fabricated prototype is depicted in Fig. 8, showing a fairly constant group delay.
3. Ultra-Wideband Balun It was mentioned in the previous section that the magnetic currents in the slots of the CPW are equal in magnitude but they are out of phase. Thus, the coupling between the microstrip lines and the slotlines of the CPW was designed such that in-phase signals are achieved at output ports of the power divider. In
9
0
S−parameter (dB)
−5 −10 −15 −20 −25 −30
S22
−35 −40
S23 0
5
10 Frequency (GHz)
15
20
Figure 7: Simulated (solid lines) and measured (dashed lines) output return loss (|S22 |) and isolation between output ports (|S23 |) for the proposed ultra-wideband T-junction power
Measured Group Delay (ns)
divider.
1.5 1 0.5 0 -0.5 -1
0
5
10
15
20
Frequency (GHz) Figure 8: Measured group delay of the fabricated prototype, showing a fairly constant group delay over the frequency band of interest.
10
Figure 9: Proposed structure for balun using CPW to CPS transition. The dimensions of the structure are as follows: wg = 0.2 mm, wc = 8 mm, and wm = 1.5 mm. These dimensions correspond to 50 Ω characteristic impedance for both CPW and microstrip lines. Other dimensions are s = 6.7 mm, d = 10 mm, and rc = rm = 2 mm.
this section we demonstrate that a simple but essential change in the structure of the power divider converts it to an out-of-phase power divider or a balun. An illustration of the proposed balun is shown in Fig. 9. The structure is identical to the ultra-wideband power divider of the previous section except that one of the slotline-to-microstrip transitions is flipped. Figure 9 also shows a zoomed view of the transitions including the currents and magnetic fields. Similar to the case of T-junction power divider, the magnetic currents in the slots of the CPW are in opposite directions. Also the magnetic fields H2
H1
and
are out of phase. However, in contrast to the T-junction power divider,
in the structure of Fig. 9 the induced currents
J1
and
J2
are out of phase. In
other words, if the structure is considered as a three-port network, it acts as an out-of-phase power divider or a balun. Fig. 10 shows the equivalent circuit model of the proposed balun. The circuit is identical to the equivalent circuit model of Fig. 4, except that the 180◦ phase difference between the output signals in the balun is modeled by adopting opposite dot conventions in the two transformers. In fact, the equivalent circuit model clearly shows how the structure can be changed to operate as an in- or out-of-phase power divider.
11
n:1
Port 2
Port 1
n:1
Port 3
Figure 10: Circuit model for the CPW to CPS transition. Note the opposite dot conventions in the two transformers modeling 180◦ out of phase magnetic currents in the slots of the CPW.
In order to validate the proposed concept, an ultra-wideband balun based on the structure of Fig. 9 is designed and numerically simulated. Rogers RT5880 material with thickness 0.51 mm, dielectric constant r = 2.2, and loss tangent of 0.0009 is used as the substrate. The dimensions of the designed structure are listed in the caption of Fig. 9. To experimentally validate the concept and simulation results a prototype of the proposed ultra-wideband balun is fabricated. Photographs of the top and bottom views of the fabricated prototype are depicted in Fig. 11. The substrate material and all dimensions of the fabricated prototype corresponds to the simulated structure. Figure 12 shows both simulated (solid lines) and measured (dashed lines) S-parameters for the proposed ultra-wideband balun. Both simulation and measurement results show almost equal values of |S21 | and |S31 | that demonstrates an equal power division (-3 dB) between ports 2 and 3. The simulated return loss is more than 14 dB and the insertion loss is less than 1 dB in the frequency band 3-15 GHz. The figure shows good agreement between the simulated and measured results at the lower 12
Figure 11: Photographs of the proposed ultra-wideband balun (a) top view (CPW side), and (b) bottom view (CPS side).
frequencies. However, there is a small discrepancy between simulation and measurement results at higher frequencies, which can be due to the imperfect SMA connectors and fabrication tolerances that are not considered in the simulations. The simulated and measured amplitude and phase differences between the two output ports (ports 2 and 3) are depicted in Fig. 13. The figure shows that the amplitude imbalance between the two ports is less than 0.5 dB over 200
the frequency band of 3-16 GHz. The figure also shows a phase imbalance of 180 ± 5◦ between the two output ports over the same frequency band. Note that, unlike the structure of in-phase power divider (Fig. 3 and 5), the structure of the proposed balun (Figs. 9 and 11) is not fully symmetric. Thus, beside the numerical errors in the simulation, or fabrication tolerances in the experimental results, the asymmetry of the structure is another factor that leads to a slightly higher phase and amplitude imbalances of the proposed balun. Measured group delay of the fabricated balun is depicted in Fig. 14, showing a fairly constant group delay over the frequency band of interest. Table 1 compares the performance of the proposed balun with similar works. In order to have a fair comparison, the most recently published ultra wideband baluns in planar technologies are used as the reference for comparison. The table shows that the proposed balun provides a relatively wider operating bandwidth, low amplitude and phase imbalances and compact size. Note that impedance transformation and 180◦ phase shift required in baluns
13
0
S−parameter (dB)
−5 −10 −15 −20 −25 −30
S11 S21 S31
−35 −40
0
5
10 Frequency (GHz)
15
20
Figure 12: Simulated (solid lines) and measured (dashed lines) reflection coefficient |S11 | and transmission coefficients |S21 | and |S31 | of the proposed ultra-wideband balun.
200
Amplitude imbalance (dB)
190 185 0
180 175 170
−1
Phase imbalance (degree)
195
1
165 0
5
10 Frequency (GHz)
15
160 20
Figure 13: Simulated (solid lines) and measured (dot lines) amplitude imbalance (blue color) and phase imbalance (green color) at two output ports of the designed balun.
14
Measured Group Delay (ns)
0.8 0.6 0.4 0.2 0 -0.2 -0.4
0
5
10
15
20
Frequency (GHz) Figure 14: Measured group delay of the fabricated balun, showing a fairly constant group delay.
is usually achieved respectively through λ/4 and λ/2 TL sections. Since these structures are intrinsically narrowband, the resulted balun is also relatively narrowband. In contrast, as shown in the previous section, the impedance transformation required in the in-phase power divider is achieved from the slotlineto-microstrip transitions rather than quarter wavelength transformers. This has led to a compact in-phase power divider with ultra-wideband performance. It is also shown in this section that the proposed in-phase power divider can be readily changed to an out-of-phase power divider in which the required 180◦ phase difference between the two output ports is achieved through the relative polarity of magnetic currents in the slots of the CPW. Therefore, the proposed out-of-phase power divider presents an ultra-wideband performance. It is worth mentioning that baluns with extremely wider operating bandwidth such as those in [14, 15] are available in the literature. However, to achieve such wide bandwidths, via holes or wire bonds are required. Furthermore, in those designs fabrication processes with micro-meter resolution were used. In contrast, the proposed balun has a simple structure and is fabricated using a conventional low-cost printed board fabrication process.
15
Table 1: Perfomance Comparison with Similar Works
Amplitude
Phase
Frequency Ref
range
fH /FL
imbal-
imbal-
ance
ance
(dB)
(deg)
(GHz)
Size (λg × λg )
[21]
3.1-10.6
3.42
Not Reported
1◦
0.45 × 0.3
[26]
1-4
4
0.5
1◦
1 × 0.5
◦
1 × 0.5
[23]
3.1-10.6
3.42
1
10
[8]
0.72-2.05
2.84
1
4.8◦
1 × 0.5
[27]
1-2.25
2.25
1
3.4◦
0.14 × 0.148
[28]
1.76-8.71
4.95
0.8
6.6
◦
Fig. 11 3-16 5.33 0.5 5◦ λg : Guided wavelength at the central operating frequency
≈ 1.35 × 0.55 0.85 × 0.5
4. Conclusion In summary, an ultra-wideband transition from CPW to a pair of microstrip lines has been presented. It has been shown that the proposed transition is able to equally split the input power to two in-phase signals, that is, the device acts as a T-junction power divider. More importantly, it has been demonstrated that the out-of-phase magnetic currents in the slots of the CPW can be utilized to achieve an out-of-phase power divider or a balun. Prototypes of both devices have been designed and validated both numerically and experimentally.
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[25] D. Pozar, Microwave Engineering, 4th Edition, John Wiley & Sons, Hoboken, NJ, USA, 2009. [26] U. T. Ahmed, A. M. Abbosh, Wideband out-of-phase power divider using coupled lines and microstrip to slotline transitions, Asia-Pacific Microw. Conf. Proceedings, APMC 2. doi:10.1109/APMC.2015.7413231. [27] H.-x. Xu, G.-m. Wang, X. Chen, T.-p. Li, Broadband Balun Using Fully Artificial Fractal-Shaped Composite Right/Left Handed Transmission Line, IEEE Microw. Wirel. Components Lett. 22 (1) (2012) 16–18. doi:10.1109/ LMWC.2011.2173929. URL http://ieeexplore.ieee.org/document/6107598/ [28] L.-p. Feng, L. Zhu, Wideband Filtering Balun on a Novel Hybrid Multimode Resonator With the Functionality of Vertical Transition, IEEE Trans. Components, Packag. Manuf. Technol. 7 (8) (2017) 1324–1330. doi:10.1109/TCPMT.2017.2658669. URL http://ieeexplore.ieee.org/document/7859321/
20
S1434-8411(18)31853-3 https://doi.org/10.1016/j.aeue.2018.08.024 AEUE 52466
To appear in:
International Journal of Electronics and Communications
Received Date: Accepted Date:
11 July 2018 18 August 2018
Please cite this article as: A.K. Horestani, Z. Shaterian, Ultra-Wideband Balun and Power Divider Using Coplanar Waveguide to Microstrip Transitions, International Journal of Electronics and Communications (2018), doi: https:// doi.org/10.1016/j.aeue.2018.08.024
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultra-Wideband Balun and Power Divider Using Coplanar Waveguide to Microstrip Transitions Ali K. Horestania , Zahra Shaterianb a Aerospace
Research Institute, Ministry of Science, Research and Technology, Tehran 1465774111, Iran b School of Electrical and Computer Engineering, College of Engineering, University of Tehran, Tehran 143995713, Iran
Abstract An ultra-wideband transition from coplanar waveguide (CPW) to a pair of microstrip lines is presented in this letter. It is shown that the proposed transition can be used to devise an ultra-wideband T-junction power divider with equally split in-phase output signals. More importantly, it is demonstrated that the outof-phase magnetic currents in the slots of the CPW can be utilized to achieve an out-of-phase power divider or a balun. Both the T-junction power divider and the balun benefit from compact size and good performance over an ultra wide frequency band from 3 to 16 GHz. Prototypes of both proposed devices are fabricated and measured to provide an experimental verification on the concept and numerically predicted features. Keywords: Power divider, balun, transition, planar structures, ultra-wideband.
1. Introduction Significant attention has been given to ultra-wideband systems due to their high communication data rate, immunity to multipath interferences and low power consumption. As a result, enhancing the operating bandwidth of passive microwave devices while avoiding complex structures and maintaining the ∗ Corresponding
Author: Ali K. Horestani Email addresses: [email protected] (Ali K. Horestani), [email protected] (Zahra Shaterian)
Preprint submitted to Journal of LATEX Templates
August 13, 2018
device miniaturization has been subject of interest in academic and industrial researches in recent years [1]. With this aim, many researches have been carried out in the realization of ultra-wideband structures using coplanar waveguide (CPW) and coplanar strip (CPS) transmission lines (TLs) because these technologies are compact, low profile, easy for fabrication and as a result cost effective. These TLs also allow series and shunt mounting of lumped active and passive components. An important difference between the two types of TLs is that CPW is an unbalanced TL, whereas the CPS is a balanced line. Thus, to integrate different microwave circuits with these two planar structures, CPW-to-CPS transitions are required. For instance in many applications a symmetric (balanced) planar antenna, such as a bow-tie or dipole antenna, needs to be fed by a coaxial cable or CPW, which are unbalanced TLs. A balun that converts a balanced signal to an unbalanced one is commonly used in such cases [2]. Power dividers are other fundamental passive components, which are widely used in a variety of the microwave and millimeter-wave systems such as antenna array feeding networks, mixers and modulators [3, 4]. In contrast to the baluns, power dividers usually provide in-phase output signals. Due to the wide application of baluns and power dividers in microwave systems, various in-phase and out-of-phase CPW-to-CPS transitions have been reported in the literature. Baluns and power dividers in [5, 6, 7, 8, 9, 10, 11] that are based on half or quarter wavelength TLs are relatively large in size and narrow band. To achieve ultra-wideband CPW-to-CPS transitions, which are required for feeding wideband or multiband antennas, several structures have been reported in the literature. However, they require multi-layer substrates [12, 13], via holes [14], or air-bridges and bond wires [15]. Metamaterial-inspired baluns and power dividers such as those reported in [16, 17, 18, 19, 20], albeit compact, are narrow band, and usually require many vias and lumped or semilumped passive components. In order to avoid the complexities associated with the fabrication procedure of multi-layer structures and air-bridges, planar baluns and power dividers were presented in [21, 22, 23]. The reported structures 2
cover an ultra-wide band, however they are only appropriate for realization in microstrip technology. In this paper, new designs for ultra-wideband T-junction power divider and balun are presented. Section 2 is devoted to the design of an ultra-wideband Tjunction power divider. It is shown that, the required impedance transformation is achieved from the slotline-to-microstrip transitions rather than quarter wavelength transformers. As a result a compact power divider with ultra-wideband performance is achieved. It is also shown in Sec. 3 that the proposed in-phase power divider can be readily changed to a balun in which the required 180-degree phase difference between the two output ports is achieved through the relative polarity of magnetic currents in the slots of the CPW. Therefore, the proposed balun presents an ultra-wideband performance. In both cases, prototypes of the designed structures are fabricated and the simulation results are experimentally 50
verified. Section 4 concludes the paper.
2. Ultra-Wideband T-Junction Power Divider This section is devoted to the design of an ultra-wideband T-junction power divider. The proposed power divider is essentially developed based on a slotlineto-microstrip transition. Thus, to make the paper smoother the first subsection provides a brief review on the literature related to the slotline-to-microstrip transition. The section then presents the ultra-wideband T-junction power divider and validates the designed structure both numerically and experimentally. 2.1. Slotline-to-Microstrip Transition Fig. 1 (a) shows an illustration of the well-known structure of a slotline-tomicrostrip transition, which was experimentally investigated in [24]. As shown in the figure, the microstrip line (in yellow color) on one side of the substrate is crossed by a slotline, which is etched in the ground plane (in orange color) of the microstrip on the back side of the substrate. A comprehensive study on different methods to extend the achievable bandwidth and flatness of the
3
frequency response of the transition was conducted in [24]. It was shown that if the slotline is extended with a quarter wavelength shorted stubs, while the microstrip is shorted to the ground plane through a via-hole, the maximum bandwidth can be achieved. The study, however, showed that a passband with superior flatness can be achieved if the microstrip and slotline are extended with quarter wavelength open and short stubs, respectively. It was shown that the performance of the transition in terms of bandwidth and flatness can be further improved if the open/short stubs are realized in a radial shape, as shown in Fig. 1 (b). The equivalent circuit model for the slotline-to-microstrip transition with the excitation on the slotline side is depicted in Fig. 2. In this model, which was developed and validated by Sch¨ uppert in [24], the characteristic impedances of the slotline and microstrip are denoted by Zs and Zm , respectively. Since the characteristic impedances of the short and open stubs can generally be different from those of the slotline and microstrip, they are denoted by Zss and Zom . The electrical length of these stubs are shown by θss and θom . The effect of the discontinuities of the shorted slotline and open microstrip stub are modeled with Lss and Com , respectively. The transformer with turns ratio n in the equivalent circuit model represents the magnetic coupling between the two TLs. It is very important to note that the characteristic impedances of the slotline and the microstrip are generally different. Thus, the required impedance matching is conducted by the magnetic coupling between the two TLs, which is properly modeled by a transformer in the equivalent circuit model. 2.2. Ultra-Wideband T-Junction Power Divider By extending the concept of slotline-to-microstrip transition of the previous subsection, a transition from CPW to a pair of microstrip lines or in other words a T-junction power divider is developed in this section. An illustration of the proposed ultra-wideband T-junction power divider is shown in Fig. 3. In this illustration, the CPW, which is excited through a coaxial connector is on the top layer of the substrate, while a pair of microstrip 4
(a)
(b)
Figure 1: Microstrip to slotline transition with (a) uniform λ/4 extensions, (b) circular stub extensions. The microstrips are illustrated with yellow color and solid lines, while dotted lines are used to indicate the boundaries of the slotlines etched in the ground plane.
Figure 2: Circuit model for the slotline-to-microstrip transition.
5
lines are on the back side of the substrate. The CPW can be considered as a pair of slotlines. Due to the symmetry of the CPW structure the power guided by the CPW is already equally divided between its slotlines. Handwritten font is used in this figure and also in the text to represent instantaneous (rather than time harmonic) field vectors. Note that, as shown in the zoomed view of Fig. 3, 100
in the CPW mode the magnetic currents H1
M1
and
M2
and also magnetic fields
◦
and H2 are 180 out of phase. However, as shown in the figure, since each
of the slotline-to-microstrip transitions is the mirror image of the other one, the induced electric currents
J1
and
J2
flowing in the microstrip lines are in phase.
That is, the structure acts as an in-phase power divider. The equivalent circuit model of the proposed power divider, which is developed based on the equivalent circuit model of a slotline-to-microstrip transition (Fig. 2) is shown in Fig. 4. In order to validate the concept, an ultra-wideband T-junction power divider is designed and numerically simulated. Also, a prototype of the device is fabricated and measured. Rogers RT5880 material with thickness 0.51 mm, dielectric constant r = 2.2, and loss tangent of 0.0009 is used as the substrate. Dimensions of the proposed T-junction power divider are given in the caption of Fig. 3. Photographs of the top and bottom views of the fabricated prototype of the ultra-wideband T-junction power divider are shown in Fig. 5. Simulated and measured reflection coefficient at port 1 and transmission coefficients |S21 | and |S31 | of the power divider are depicted in Fig. 6. The solid lines show the simulation results, whereas the experimental results are shown by dashed lines. The figure shows that the simulated insertion loss is less than 1 dB across a wide frequency band from 3 to 16 GHz. The figure also shows that a simulated return loss better than 15 dB is achieved over the frequency band of 3 to 16 GHz. This wideband operation is due to the strong coupling between each slot of the CPW and the corresponding microstrip line, acting as a wideband impedance transformer. Note that in a conventional T-junction power divider, matching at the input port with a source impedance of 50 Ω is achieved when the two output transmission lines are 100 Ω lines. Thus, quarter6
Figure 3: Layout of the proposed ultra-wideband T-junction power divider. The dimensions of the structure are: wg = 0.2 mm, wc = 4.3 mm, and wm = 1.5 mm (corresponding to 50 Ω lines). Other dimensions are: s = 4.5 mm, d = 10 mm, and rm = rc = 2 mm.
Com θom , Zom
Zs Lss
θss , Zss n:1
Port 1
θom , Zom
Zs Lss
Zm
Port 2
Com
Zm
θss , Zss n:1
Figure 4: Circuit model for the T-junction power divider.
7
Port 3
Figure 5: Photographs of a fabricated prototype of the proposed ultra-wideband T-junction power divider (a) top view, and (b) bottom view.
wave transformers have to be used to bring the impedances of the output lines back to the desired levels [25]. The disadvantage of this method is that the operating bandwidth of the power divider will be limited to the bandwidth that the quarter-wave transformers can provide [25]. In contrast, in the proposed T-junction power divider 50 Ω TLs are used for all three ports of the structure. The reason is that in the proposed structure the impedance transformation is carried out by the coupling between the CPW and microstrip lines. This feature, which is modeled by a pair of transformers in the equivalent circuit model of Fig.4, bypasses the need for quarter-wave transformers, and results in an ultra-wideband performance. Figure 6 shows that the insertion loss at frequencies higher than 14 GHz is slightly higher than expected. This loss can be attributed to free-space radiation and surface wave losses, which are more pronounced at higher frequencies. The figure also shows that the measured insertion loss at frequencies higher than 14 GHz is slightly higher than the simulated one. This discrepancy most probably arises from the fabrication tolerances and imperfect SMA connectors at those high frequencies.
The simulated (solid lines) and measured (dashed lines)
reflection from output port (|S22 |) and the isolation between the output ports (|S23 |) are depicted in Fig. 7. The figure shows that return loss at the output ports is better than 6 dB, while an isolation around 10 dB is achieved between the output ports. Note that this level of isolation is typical of a T-junction power
8
0
S−parameter (dB)
−5 −10 −15 −20 −25
S11
−30
S21
−35
S31
−40
0
5
10 Frequency (GHz)
15
20
Figure 6: Simulated (solid lines) and measured (dashed lines) input reflection coefficient (|S11 |) and transmission coefficients (|S21 | and |S31 |) of the proposed ultra-wideband T-junction power divider.
dividers, which are lossless three-port structures [25]. Increasing the isolation and return loss at the output ports by introducing lossy components to form a Wilkinson power divider can be the subject of a future study. It is also im150
portant to note that the structure is completely symmetric. Thus, theoretically the output signals must be identical both in magnitude and phase. Therefore, any phase difference in numerical or experimental results has to be assigned to numerical errors and imperfect fabrication of the structure. Measured group delay of the fabricated prototype is depicted in Fig. 8, showing a fairly constant group delay.
3. Ultra-Wideband Balun It was mentioned in the previous section that the magnetic currents in the slots of the CPW are equal in magnitude but they are out of phase. Thus, the coupling between the microstrip lines and the slotlines of the CPW was designed such that in-phase signals are achieved at output ports of the power divider. In
9
0
S−parameter (dB)
−5 −10 −15 −20 −25 −30
S22
−35 −40
S23 0
5
10 Frequency (GHz)
15
20
Figure 7: Simulated (solid lines) and measured (dashed lines) output return loss (|S22 |) and isolation between output ports (|S23 |) for the proposed ultra-wideband T-junction power
Measured Group Delay (ns)
divider.
1.5 1 0.5 0 -0.5 -1
0
5
10
15
20
Frequency (GHz) Figure 8: Measured group delay of the fabricated prototype, showing a fairly constant group delay over the frequency band of interest.
10
Figure 9: Proposed structure for balun using CPW to CPS transition. The dimensions of the structure are as follows: wg = 0.2 mm, wc = 8 mm, and wm = 1.5 mm. These dimensions correspond to 50 Ω characteristic impedance for both CPW and microstrip lines. Other dimensions are s = 6.7 mm, d = 10 mm, and rc = rm = 2 mm.
this section we demonstrate that a simple but essential change in the structure of the power divider converts it to an out-of-phase power divider or a balun. An illustration of the proposed balun is shown in Fig. 9. The structure is identical to the ultra-wideband power divider of the previous section except that one of the slotline-to-microstrip transitions is flipped. Figure 9 also shows a zoomed view of the transitions including the currents and magnetic fields. Similar to the case of T-junction power divider, the magnetic currents in the slots of the CPW are in opposite directions. Also the magnetic fields H2
H1
and
are out of phase. However, in contrast to the T-junction power divider,
in the structure of Fig. 9 the induced currents
J1
and
J2
are out of phase. In
other words, if the structure is considered as a three-port network, it acts as an out-of-phase power divider or a balun. Fig. 10 shows the equivalent circuit model of the proposed balun. The circuit is identical to the equivalent circuit model of Fig. 4, except that the 180◦ phase difference between the output signals in the balun is modeled by adopting opposite dot conventions in the two transformers. In fact, the equivalent circuit model clearly shows how the structure can be changed to operate as an in- or out-of-phase power divider.
11
n:1
Port 2
Port 1
n:1
Port 3
Figure 10: Circuit model for the CPW to CPS transition. Note the opposite dot conventions in the two transformers modeling 180◦ out of phase magnetic currents in the slots of the CPW.
In order to validate the proposed concept, an ultra-wideband balun based on the structure of Fig. 9 is designed and numerically simulated. Rogers RT5880 material with thickness 0.51 mm, dielectric constant r = 2.2, and loss tangent of 0.0009 is used as the substrate. The dimensions of the designed structure are listed in the caption of Fig. 9. To experimentally validate the concept and simulation results a prototype of the proposed ultra-wideband balun is fabricated. Photographs of the top and bottom views of the fabricated prototype are depicted in Fig. 11. The substrate material and all dimensions of the fabricated prototype corresponds to the simulated structure. Figure 12 shows both simulated (solid lines) and measured (dashed lines) S-parameters for the proposed ultra-wideband balun. Both simulation and measurement results show almost equal values of |S21 | and |S31 | that demonstrates an equal power division (-3 dB) between ports 2 and 3. The simulated return loss is more than 14 dB and the insertion loss is less than 1 dB in the frequency band 3-15 GHz. The figure shows good agreement between the simulated and measured results at the lower 12
Figure 11: Photographs of the proposed ultra-wideband balun (a) top view (CPW side), and (b) bottom view (CPS side).
frequencies. However, there is a small discrepancy between simulation and measurement results at higher frequencies, which can be due to the imperfect SMA connectors and fabrication tolerances that are not considered in the simulations. The simulated and measured amplitude and phase differences between the two output ports (ports 2 and 3) are depicted in Fig. 13. The figure shows that the amplitude imbalance between the two ports is less than 0.5 dB over 200
the frequency band of 3-16 GHz. The figure also shows a phase imbalance of 180 ± 5◦ between the two output ports over the same frequency band. Note that, unlike the structure of in-phase power divider (Fig. 3 and 5), the structure of the proposed balun (Figs. 9 and 11) is not fully symmetric. Thus, beside the numerical errors in the simulation, or fabrication tolerances in the experimental results, the asymmetry of the structure is another factor that leads to a slightly higher phase and amplitude imbalances of the proposed balun. Measured group delay of the fabricated balun is depicted in Fig. 14, showing a fairly constant group delay over the frequency band of interest. Table 1 compares the performance of the proposed balun with similar works. In order to have a fair comparison, the most recently published ultra wideband baluns in planar technologies are used as the reference for comparison. The table shows that the proposed balun provides a relatively wider operating bandwidth, low amplitude and phase imbalances and compact size. Note that impedance transformation and 180◦ phase shift required in baluns
13
0
S−parameter (dB)
−5 −10 −15 −20 −25 −30
S11 S21 S31
−35 −40
0
5
10 Frequency (GHz)
15
20
Figure 12: Simulated (solid lines) and measured (dashed lines) reflection coefficient |S11 | and transmission coefficients |S21 | and |S31 | of the proposed ultra-wideband balun.
200
Amplitude imbalance (dB)
190 185 0
180 175 170
−1
Phase imbalance (degree)
195
1
165 0
5
10 Frequency (GHz)
15
160 20
Figure 13: Simulated (solid lines) and measured (dot lines) amplitude imbalance (blue color) and phase imbalance (green color) at two output ports of the designed balun.
14
Measured Group Delay (ns)
0.8 0.6 0.4 0.2 0 -0.2 -0.4
0
5
10
15
20
Frequency (GHz) Figure 14: Measured group delay of the fabricated balun, showing a fairly constant group delay.
is usually achieved respectively through λ/4 and λ/2 TL sections. Since these structures are intrinsically narrowband, the resulted balun is also relatively narrowband. In contrast, as shown in the previous section, the impedance transformation required in the in-phase power divider is achieved from the slotlineto-microstrip transitions rather than quarter wavelength transformers. This has led to a compact in-phase power divider with ultra-wideband performance. It is also shown in this section that the proposed in-phase power divider can be readily changed to an out-of-phase power divider in which the required 180◦ phase difference between the two output ports is achieved through the relative polarity of magnetic currents in the slots of the CPW. Therefore, the proposed out-of-phase power divider presents an ultra-wideband performance. It is worth mentioning that baluns with extremely wider operating bandwidth such as those in [14, 15] are available in the literature. However, to achieve such wide bandwidths, via holes or wire bonds are required. Furthermore, in those designs fabrication processes with micro-meter resolution were used. In contrast, the proposed balun has a simple structure and is fabricated using a conventional low-cost printed board fabrication process.
15
Table 1: Perfomance Comparison with Similar Works
Amplitude
Phase
Frequency Ref
range
fH /FL
imbal-
imbal-
ance
ance
(dB)
(deg)
(GHz)
Size (λg × λg )
[21]
3.1-10.6
3.42
Not Reported
1◦
0.45 × 0.3
[26]
1-4
4
0.5
1◦
1 × 0.5
◦
1 × 0.5
[23]
3.1-10.6
3.42
1
10
[8]
0.72-2.05
2.84
1
4.8◦
1 × 0.5
[27]
1-2.25
2.25
1
3.4◦
0.14 × 0.148
[28]
1.76-8.71
4.95
0.8
6.6
◦
Fig. 11 3-16 5.33 0.5 5◦ λg : Guided wavelength at the central operating frequency
≈ 1.35 × 0.55 0.85 × 0.5
4. Conclusion In summary, an ultra-wideband transition from CPW to a pair of microstrip lines has been presented. It has been shown that the proposed transition is able to equally split the input power to two in-phase signals, that is, the device acts as a T-junction power divider. More importantly, it has been demonstrated that the out-of-phase magnetic currents in the slots of the CPW can be utilized to achieve an out-of-phase power divider or a balun. Prototypes of both devices have been designed and validated both numerically and experimentally.
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