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A Novel LTCC Miniaturized Dualband Balun Yong-Xin Guo, Senior Member, IEEE, Z. Y. Zhang, L. C. Ong, Senior Member, IEEE, and M. Y. W. Chia, Member, IEEE
Abstract—In this letter, a novel low temperature cofired ceramic (LTCC) miniaturized dualband balun using a tapered line structure has been presented. The key concept for our dualband solution is by employing tapered line structures in the conventional Marchand baluns. In our new design, we have demonstrated the tapered line structure can shift the periodic operation frequency to a desired frequency band. A sample balun design has been presented. The novel LTCC dualband balun has been simulated and verified by measurement. The measured results exhibit that the proposed balun can easily cover the important radio frequency bands at 2.4 GHz, 5.25 GHz, and 5.85 GHz. Index Terms—Balun, dualband balun, low temperature cofired ceramic (LTCC), Marchand balun, planar balun.
I. INTRODUCTION
A
BALUN is a device for converting signals between an unbalanced circuit structure and a balanced circuit structure. The signal of a balanced circuit structure comprises two signal components with the same magnitude but at 180 phase difference. Many analog circuits require balanced inputs and outputs in order to reduce noise and high order harmonics as well as improve the dynamic range of the circuits [1]–[3]. Baluns are key components in many wireless communication systems for realizing components such as mixer, amplifier, multipliers, and balanced antennas. There are several types of baluns that are either active or passives. For the active balun, the transistor may introduce the noise and exhaust more energy [4]. Passive baluns can be classified as lumped-type, coil-type, and distributed-type baluns [5]–[7]. The advantages of a lumped-type balun are small volume and light weight. However, it is not easy to maintain 180 phase difference and identical magnitude between the two signals. Coil-type baluns have been widely used in lower frequency and ultra high frequency (UHF) bands. When a coil-type balun is used in higher than the UHF band, it usually has a drawback of having considerable loss. Distributed-type baluns can further be classified as a 180 hybrid balun and a Marchand balun. A 180 hybrid balun has a fairly good frequency response in the microwave frequency band. However, its size often poses a problem when it is used in the radio frequency range between 200 MHz and several gigahertz. A Marchand balun commonly used in the industry comprises two sections of quarter-wave coupled lines [8]. This type of baluns has a fairly large bandwidth. Both phase difference and power distribution of a Marchand balun are reasonably good. However, the Marchand balun Manuscript received September 13, 2005; revised November 17, 2005. This work was supported by SERC, A*STAR, Singapore. The authors are with the Institute for Infocomm Research, Singapore 117674 (e-mail:
[email protected]). Digital Object Identifier 10.1109/LMWC.2006.869795
consisting of two identical 4 coupled lines still occupies a big area, especially at low frequencies. Efforts have been taken to reduce the size of conventional Marchand baluns to meet the requirements, e.g., low-cost and small-size, for wireless communications at radio frequency (RF) frequencies. The use of low temperature co-fired ceramic (LTCC) technology in RF circuit design has become very popular due to its high performance, high integration density, and high reliability. LTCC is a multiplayer ceramic technology which provides an ability to embed passive components in layers while the active elements are mounted on the surface layer. On the other hand, in prior arts, miniature Marchand baluns or lumped-typed baluns implemented in multilayer structure are in single narrow-band operation [9], [10]. In this letter, a novel miniature dual-band Marchand balun is presented for the first time. In our proposed new balun structure, the two sections of the 4 transmission line in the conventional Marchand balun have been replaced by the tapered line structures. The idea of using tapered coupling lines in this Marchand balun comes from the filter designs with the well-known tapered transmission line resonators [11]. Typically, the center frequency of the second passband with uniform quarter-wavelength line resonators is three times that of the fundamental frequency. With the use of nonuniform line resonators, the spurious resonance frequency response can be controlled by the tapered line characteristic [11]. In this work, the tapered line resonator is employed in the balun design as the basic coupling element, the spurious response can shift from 3.0 to 2.3. A sample dual-band LTCC balun operating at 2.4/5.6 GHz has been designed, simulated, fabricated, and measured. Good agreement is obtained between prediction and measurement. II. STRUCTURE DESCRIPTION Fig. 1(a) and (b) show the schematic of the conventional Marchand balun and the proposed dual-band tapered balun, respectively. The conventional Marchand balun consists of two 4 coupled lines. Since the symmetrical 4 symmetrical coupled line has a periodic characteristic, the second operation band will appear at the three times frequency point of the first band. By applying the tapered lines to the conventional Marchand balun, the periodic characteristic will change. The chip-type balun is designed to cover the frequency bands 2.4 GHz, 5.25 GHz, and 5.85 GHz. The unbalanced input port impedance and each of the balanced output ports impedances for convenience. The design procedures are set to be 50 are as follows. First, a conventional Marchand balun operating at around 2 GHz is designed, and accordingly another operation frequency will appear at 6 GHz. Second, the symmetrical coupled line sections in the conventional Marchand balun are
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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 16, NO. 3, MARCH 2006
Fig. 1. Schematic of the baluns (a) conventional Marchand balun and (b) proposed dual-band tapered balun.
Fig. 3. Amplitude responses of the proposed LTCC dual-band tapered balun, the dashed lines stand for the measured results and the solid lines stand for the simulated results: (a) at the lower band and (b) at the upper band.
Fig. 2. Three-dimensional structure of the proposed dual-band tapered balun in LTCC implementation.
replaced by two unfolded tapered coupled line sections. Third, we use the commercial electromagnetic simulation software IE3D to tune and get the physical dimensions of the proposed balun. Finally, the coupled lines are spiraled to shrink the balun further. A novel LTCC tapered balun has been developed. The central idea is based on that the tapered line can shift the periodical frequency band to the desired frequency point. The first operation band has been carefully designed to be at 2.4 GHz. For the conventional case, the second operation band will appear at three times of the first operation band 7.2 GHz. While in our case, the second operation band is at around 5.6 GHz. Notice that the application of the tapered coupled line on the conventional Marchand balun moves the next periodical band from 3 to approximately 2.3 (5.6 GHz/2.4 GHz) in the particular design. Fig. 2 shows the three-dimensional (3-D) structure of the proposed LTCC dual-band tapered balun. The LTCC dual-band tapered balun comprises a laminate formed by eight dielectric substrates superimposed one on the other. Two ground electrodes are formed on a main surface of the first and the eighth
dielectric substrate, respectively. The structure of the balun is broadside coupled and both of the tapered coupled line sections are of meandered shape to minimize the occupied size. Since the balun structure is symmetrical, a ground plane is placed at the middle layer of the structure separates these two coupled line sections. The three ground planes in the balun structure are connected with each other through via holes. The central ground plane can shield the upper and lower parts from the influence of each other. The connection between different layers is realized by via holes through the ceramic chip. In practice, the unbalanced and balanced ports are extended to the top layer for measurement using proper designed transmission lines. The tapered dual-band balun is realized in a commercial LTCC technology utilizing standard 3.7-mil-thick FERRO A6-M ceramic tapes with a dielectric constant of 5.9. The tapered line has a continuously changing width from 6 to 13 mils. The fabricated balun has an area of 2.6 mm 2.6 mm and a thickness of 0.75 mm. The size can reduced further if high permittivity materials and size-reduction techniques are employed. III. RESULTS AND DISCUSSIONS Figs. 3 and 4(a) and (b) display the simulated and measured results for the amplitude and phase balance responses of the proposed LTCC dual-band tapered balun, respectively. The dashed lines stand for the measured results and the solid lines stand for the simulated results. The new balun was simulated using IE3D.
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respectively. And at the upper band from 5.2 to 6 GHz, the measured amplitude and phase imbalance between the two balanced output ports are within 1.2 dB and 10 , respectively, which may be due to the fabrication tolerance. Referring to Figs. 3 and 4, it is seen that the center frequency of the second passband of the proposed dualband balun with nonuniform quarter-wavelength line resonators has shifted as expected. The spurious response can be controlled by changing the ratio of the width of the taper lines. IV. CONCLUSION A novel LTCC miniaturized dualband balun using a tapered line structure has been presented in this letter. In our new design, we have demonstrated the tapered line structure can shift the periodic operation frequency to a desired band. A sample balun design has been presented. The novel LTCC dualband balun has been simulated and verified by measurement. The measured results exhibit that the proposed balun can easily cover the important RF frequency bands 2.4 GHz, 5.25 GHz, and 5.85 GHz. REFERENCES
Fig. 4. Amplitude and phase imbalance responses of the proposed LTCC dualband tapered balun, the dashed lines stand for the measured results and the solid lines stand for the simulated results: (a) at the lower band and (b) at the upper band.
The measurement was carried out on an HP8510C vector network analyzer. It is observed that good agreement is achieved between measurement and simulation. At the lower band, the measured return loss is found to be better than 15 dB from 2.35 to 2.55 GHz and better than 10 dB over the frequency band from 2.1 to 2.6 GHz, or around 21.3% bandwidth. At the upper band, the measured return loss is found to be better than 10 dB across the frequency band from 5.2 to 6 GHz. At the lower band around 2.4 GHz, the measured amplitude and phase imbalance between the two balanced output ports are within 1 dB and 4 ,
[1] R. Bawer and J. J. Wolfe, “A printed-circuit Balun for use with spiral antennas,” IRE Trans. Microw. Theory Tech., vol. MTT-8, no. 3, pp. 319–325, May 1960. [2] B. R. Hallfold, “A designer’s guide to planar mixer baluns,” Microwaves, vol. 18, pp. 52–57, Dec. 1979. [3] R. Bassett, “Three Balun designs for push-pull amplifiers,” Microwaves, vol. 19, pp. 47–52, Jul. 1980. [4] M. Rajashekharaiah, P. Upadhyaya, and H. Deukhyoun, “A compact 5.6 GHz low noise amplifier with new on-chip gain controllable active balun,” in Proc. IEEE Workshop Microelectron. Electron Devices, 2004, pp. 131–132. [5] Y. J. Yoon, Y. Lu, R. C. Frye, M. Y. Lau, P. R. Smith, L. Ahlquist, and D. P. Kossives, “Design and characterization of multiplayer spiral transmission-line baluns,” IEEE Trans. Microw. Theory Tech., vol. 47, no. 9, pp. 1841–1847, Sep. 1999. [6] B. A. Munk, Balun, etc., 3rd ed, J. D. Kraus and R. J. Marhefka, Eds. New York: McGraw-Hill, 2002, ch. 23. [7] C. Cho and K. C. Gupta, “A new design procedure for single-layer and two-layer three-line baluns,” IEEE Trans. Microw. Theory Tech., vol. 46, no. 12, pp. 2514–2519, Dec. 1998. [8] N. Marchand, “Transmission-Line Conversion Transformers,” Electronics, vol. 17, pp. 142–146, Dec. 1944. [9] C. W. Tang, J. W. Sheen, and C. Y. Chang, “Chip-type LTCC-MLC baluns using the stepped impedance method,” IEEE Trans. Microw. Theory Tech., vol. 45, no. 12, pp. 2150–2152, Dec. 1997. [10] J. W. Sheen and C. W. Tang, “LTCC-MLC balun for WLAN/Bluetooth,” in IEEE MTT-S Int. Dig., 2001, pp. 315–318. [11] M. Makimoto and S. Yamashita, “Bandpass filters using parallel coupled stripline stepped impedance resonators,” IEEE Trans Microw. Theory Tech., vol. MTT-28, no. 12, pp. 1413–1417, Dec. 1980.
