Planar Compact Dual-Band Coupled-Line Balun with High Isolation

Weiwei Zhang, Yongle Wu, Weimin Wang, Xiaochuan Shen

Beijing Key Laboratory of Work Safety Intelligent Monitoring, School of Electronic Engineering,Beijing University of Posts and Telecommunications, Beijing 100876, China

* The corresponding author, email: clarence.zhang11@gmail.com

I. INTRODUCTION

Balun [1], which is used to convert signals between the unbalanced circuit and the balanced circuit, plays a key role in the modern wireless communication systems. According to the reported paper, balun has been successfully applied to the wideband balanced frequency doubler [2], balanced mixers [3], [4], loop and balanced antennas [5], [6], push-pull microwave power amplifiers (PAs) [7], [8], and the radio frequency (RF) receivers [9], [10], etc.On the other side, dual-band balun, as a hot topic for the multiband and multimode wireless communication, has been researched for many years. According to the circuit structures, they can be classified into four types,namely, Marchand balun [11]-[14], branchline balun [15]-[19], coupled-line balun [20]-[23], and balun based on the lumped components [24]-[25], etc.

However, there are still some problems needed to be solved. For example, the Marchand dual-band balun [11]-[14] require very tight-coupling coupled lines, which cannot be easily realized by the planar transmission lines because of the limitation of the gaps between two microstrip lines. The methods of low temperature co- fired ceramic (LTCC) technology[11], multilayer broadside-coupled coplanar striplines [12], the modified structure based on capacitors and open stubs [13], and multilayer symmetrical striplines [14] can be used to solve the tight-coupling problems, however,the complex of the design increases correspondingly. For the branch-line structure, it needs 7 [15], 14 [16], 16 [17]-[19] microstrip lines to design a dual-band balun respectively,which makes the circuit very large.

In addition, the coupled-line structure [20],which consists of multiple coupled units needed to be tuned carefully, is very inefficiency.One another dual-band balun [21], which is based on the stepped-impedance coupled-line resonators, is realized by the striplines technology and it belongs to the multilayer structure. [22] proposed a compact balun, which is based on two coupled lines and one open stub, however the isolation performance is not taken into consideration. Though one dual-band coupled-line balun with high isolation is proposed in [23], the circuit structure is very complex due to four pairs of coupled lines, six transmission-line stubs, and one resistor. As for the fourth type, the performance [24], [25]will deteriorate greatly with the increment of the operating frequency because of the parasitic effect of the lumped components.

It can also be concluded that almost all the dual-band baluns, except [19] and [23], fail to take the isolation between the two outputs,and outputs matching into consideration. The isolation is very important because it can prevent the signals of one output from leaking into the other output, and vice versa. In addition, a coupled-line balun with high isolation has been proposed in [26], however it can only work at one frequency.

Fig.1 The circuit structure of the proposed dual-band balun

Given the analysis above, a planar coupled-line circuit as shown in Fig. 1 is proposed and this balun features: 1) dual-band operation and high isolation between two outputs, 2) all ports matching and out-of-phase performance at the operating frequency, 3) very compact structure because of the coupled lines, 4)planar circuit structure for the printed circuit board (PCB) technology. The remainders are organized as follows. The circuit structure and even-(odd-) mode analysis are given in Section II. The available frequency ratios are discussed concretely in Section III. A practical experiment, which is fabricated and measured to validate the circuit theory, and the comparison with state-of-the-art balun is in Section IV,and a conclusion is drawn at last.

II. DESIGN THEORY OF THE PROPOSED DUAL-BAND BALUN

The structure of the dual-band balun with high isolation is depicted in Fig.1. It can be seen that this circuit consists of three cascaded pairs of coupled lines in the center, one tapped open stub Z4on the left side, and one grounded resistor R0for outputs matching and isolation on the right side. Essentially, the even-(odd-) mode impedance of the coupled lines is Zei(Zoi), where i=1, 2, and 3. All the electrical lengths of the transmission lines are set to be θ to guarantee the dual-band performance and they are different from those in [26]. In addition, the source-(load-) impedance is defined as RS(RL), which indicates that this balun possesses the inherent impedance transformation.

The even-(odd-) analytical method [26] can be used to analyze this asymmetrical circuit because it can be seen as the corresponding symmetrical four-port circuit with one port open. The sufficient and necessary conditions for all ports matching, equal power division with out of phase, and high outputs isolation are listed as follows:

where the subscript e represents the even mode, and o represents the odd mode.

2.1 Even-mode analysis

When even-mode signal is excited, the impedance of the tapped open stub Z4and the isola-tion resistance R0are the double of the original values as shown in Fig. 2 (a). The stepped impedances (2Z4and Ze1) generate two transmission zeros at f1and f2under Equation (2) to satisfy the condition (1a). Then the even-mode circuit can be simplified into Fig.2 (b).

According to the transmission-line theory and the ABCD transmission matrix, Equation(3) is employed to make the port 2 be matched under the even mode, namely,

(3)以上两条工艺路线都受限攀枝花钛原料的固有特性，通过选矿可以选别其中已解离的硅酸盐相，从而大幅降低产品中CaO、SiO2含量。因此，攀枝花钛原料开发应用于富钛料，后期研究方向可考虑选矿工艺的调整和优化。

where

After substituting the ABCD parameters into Equation (3), the even-mode impedance of the second coupled line Ze2and the isolation resistance R0can be obtained as

2.2 Odd-mode analysis

When the odd-mode signal is excited, the tapped open stub Z4and the isolation resistor R0are shorted. Then the equivalent odd-mode circuit is depicted in Fig.3. The return loss(S11o) of the port 1 under the odd mode is derived as:

where

Fig.2 (a) The equivalent, and (b) the simplified even-mode sub-circuit

Fig.3 Odd-mode equivalent circuit of the dual-band balun

By solving Equations (6) and (7) simultaneously, the closed-form design equations about the odd-mode impedances of the first and the third coupled lines (Zo1and Zo3) can be obtained as (8).

2.3 Dual-band theoretical support

It can be seen that the closed-form Equations(2), (5), and (8) are all the even functions of the electrical length θ and it is the theoretical foundation of the dual-band performance. The electrical lengths at f1and f2can be defined as:

where

III. ANALYSIS OF THE FREQUENCY RATIOS

Table I The circuit parameters when g is in the range from 1.7 to 2.9

Then, the available frequency ratios g from 1.7 to 2.9 and the corresponding circuit parameters are listed in Table I. Actually, there are many solutions at each frequency ratio g,and only one solution is listed here.

In addition, the minimum frequency ratio(g=1.7) as Ex A, and maximum frequency ratio (g=2.9) as Ex B are adopted to verify that this balun can operate normally. Thefirst operating frequency is set to be 1 GHz, and other parameters can be found in Table I. The corresponding calculated scattering parameters of Ex A (B) are plotted in Fig. 4.

It can be seen that the insertion loss (|S21|and |S31|) are all 3.01 dB at the operating frequency in Fig. 4 (a) and (b), indicating the equal power division. All the refection coefficients (|S11|, |S22|, and |S33|) are all below -45 dB, which indicates three ports (one input and two outputs) match very well. In addition, the isolation between two outputs (|S23|) is below-45 dB because of the grounded resistor. Furthermore, the phase difference between two outputs is out-of-phase and it is not plotted in Fig. 4.

IV. MEASUREMENT AND COMPARISON

4.1 Experiment and discussion

In this section, a planar dual-band coupled-line balun, which operates at 0.9/1.8GHz,is designed to verify the correctness of the circuit theory. The circuit parameters are calculated and obtained as: RS=30 Ω, RL=50 Ω,Ze1=70 Ω, Zo1=49.40 Ω, Ze2=34.20 Ω, Zo2=30 Ω,Ze3=32.30 Ω, Zo3=32.30 Ω, Z4=105 Ω, θ=60o,and R0=65.21 Ω. The ideal scattering parameters and phase difference between two outputs of this experiment are calculated using the Advanced Design System (ADS) from Keysight Technology and plotted in Fig. 5.

It can be observed from Fig. 5 that this balun has the equal power division with out of phase. The insertion loss (|S21| and |S31|) are both equal to 3.01 dB and the phase difference is 180oat 0.9 GHz (1.8 GHz), simultaneously.The magnitudes of other scattering parameters(|S11|, |S22|, |S33|, and |S23|) are all below -40 dB,which indicates that this balun has perfect ports matching and excellent output isolation.

This balun is fabricated on the substrate Rogers 4350B with a relatively dielectric constant of 3.48 and a thickness of 0.762 mm to verify its feasibility in practical. It should be worth noting that the source impedance (port 1) is not 50-Ω and one dual-band impedance transformer with impedances (ZT1=42.15Ω,and ZT2=35.58Ω) [27] should be added in order to be measured by the Vector Network Analyzer directly. The final top view of the fabricated balun is shown as in Fig. 6, and it consists of the main body and one dual-band impedance transformer.

The simulated results are optimized by the High Frequency Structure Simulator (HFSS)from the Ansys and the final physical circuit dimensions (units: mm) are shown in Fig. 7.The circuit size of the main body is only 0.024λ2g, which means this circuit is very compact. It can be observed that the simulated results obtained from the HFSS and the measured results got from the VNA ZVA8 agree very well in Fig. 8. Table II lists the scattering parameters at 0.9/1.8GHz to compare the simulated and measured results better.

Fig.4 The calculated scattering parameters when the frequency ratio is (a) g=1.7, and (b) g=2.9

Fig.5 (a) The magnitude of the calculated scattering parameters, and (b) the phase difference between two outputs of the experiment

Fig.6 Top view of the fabricated dual-band coupled-line balun

Fig.7 Layout of the proposed coupled-line balun

The simulated magnitude difference between the insertion loss (|S21| and |S31|) is only 0.1 dB (0.26 dB) at 0.9 GHz (0.8 GHz), and the measured magnitude difference is only 0.072 dB (0.1 dB), indicating the equal power division. The simulated phase difference between two outputs is 180.58o(181.38o) at 0.9 GHz (0.8 GHz), and the measured phase difference is 179.97o(182.80o), which means that the signals at two outputs are out-of-phase.The re flection coefficients (|S11|, |S22|, and |S33|)are all below -19 dB (-15 dB) according to the simulated (measured) results. The isolation between two outputs, as an important parameter for the proposed balun, is enhanced by one grounded resistor in this paper. It can be seen that the measured isolation (|S23|) is below -24 dB.

In general, the measured results match very well with the simulated results in Fig. 8, and they can fully validate the correctness of the circuit structure and the design theory in Section II. Though there still exits small performance degradation, which may be caused by the substrate loss, manufacture errors, and so on, the measured results are acceptable on the whole.

4.2 Comparison with the state-ofthe-art dual-band balun

As compared with the state-of-the-art dual-band balun in Table III, the factors, including the fabrication technology, the layer, the closed-form design theory, and the performance of isolation between two outputs, are very important to realize a dual-band balun in practical. The Marchand balun [11]-[14],as a traditional method, requires tight-coupling coupled lines. In addition, the Marchand baluns do not possess the performance of output isolation. Though the branch-line baluns [15]-[18] can be realized by the microstrip technology, they occupy large circuit size and they do not have the performance of output isolation. Only the branch-line balun[19] possesses the output isolation by using one grounded resistor, however, 16 microstrip lines, which are used to design the dual-band balun, make the circuit very large. Similarly,the coupled-line baluns [20]-[22] do not have the performance of high isolation between two outputs. The circuit structure of the coupled-line balun [23] is very complex because it is based on four pairs of coupled lines, six transmission-line stubs, and one resistor. The performance of the balun [24]-[25], which is based on the lumped components, will deteriorate with the increasing of the operating frequency.

It can be concluded that the proposed coupled-line dual-band balun features the planar structure, inherent impedance transformation,closed-form equation, and high isolation.

V. CONCLUSION

A planar dual-band balun is proposed in this paper. This proposed circuit, which is composed of three pairs of coupled lines, one tapped open stub, and one isolation resistor,features compact structure, dual-band performance, equal power division with out of phase, good ports matching, and high isolation. The circuit is analyzed by using the traditional even-(odd-) mode method, and the closed-form solutions are obtained. The measured magnitude difference between the insertion loss (|S21|, and |S31|) are only 0.072 dB and 0.1 dB at two operating frequencies,meanwhile, the phase difference are 179.97o,and 182.80o, respectively. It can be believed that this proposed dual-band balun can be used in the balanced mixers, push-pull amplifiers,and other balanced systems.

ACKNOWLEDGEMENTS

This work was fully supported by National Natural Science Foundations of China(No.61422103, and No.61671084), National Key Basic Research Program of China (973 Program) (No.2014CB339900), BUPT Excellent Ph.D. Students Foundation (CX2016303),and China Scholarship Council.

Table III The comparison with the state-of-the-art publications

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