TECHNICAL FEATURE

Design of a Planar Microstrip Balun at S-band

An innovation for the Marchand balun model is presented. The structure of the designed planar microstrip matching circuit is fairly simple, practical and easily fabricated without requiring complex multiple metal or dielectric layers. Moreover, the designed balun performs well. The measured phase difference between the balance ports was within 180° ±3° from 1.9 to 3.9 GHz and the magnitudes of S₂₁ and S₃₁ are fairly close to ­3 dB from 2.4 to 3.2 GHz. The design and simulation of the planar microstrip balun show good agreement with the measured results.

Jwo-Shiun Sun and Tsung-Lin Lee
National Taipei University of Technology
Department of Electronic Engineering
Taipei, Taiwan, ROC

A balun, an unbalanced to balanced line transformer, was first proposed by Marchand¹ in 1944. Because of its output-balanced characteristic with a phase difference of 180°, it is a significant component for many different applications, including mixers,^2­6 balanced amplifiers^7­9 and antennas.^10­14 Because of their wide bandwidth and inherent outputs of equal amplitude with 180° phase difference, many different implementations of baluns such as log periodic baluns¹⁵ , three line baluns,¹⁶ N-section half-wave baluns,¹⁷ microstrip taper baluns18 and lumped element baluns^19­20 are used in microwave circuits.

A balun is a circuit that transforms from an unbalanced input to balanced outputs or vice-versa. It is also an impedance transformer that can produce two different signal levels with 180° phase difference. Generally, baluns can be classified into active or passive types. The design criteria for baluns are better isolation, impedance matching, balanced to unbalanced transformation and lower noise interaction in-between ports.

Active baluns^13­16 are constructed with active elements (FETs) with a common source or common gate configuration to achieve impedance matching. Generally, an active balun with output gain can decrease an oscillator's input gain requirement in mixer applications. However, a FET is an active device with DC power dissipation and higher noise level. Passive baluns are constructed with coplanar transmission lines, coaxial lines or microstrip lines. Passive baluns are widely used in either mixers or power amplifiers. With the advantage of a compact configuration combining two identical coupled lines with different termination, a passive structure balun, with 180° phase difference at S-band (2­4 GHz) of the Marchand model, is studied in this article.

RESULTS AND DISCUSSIONS

Figure 1 shows a typical Marchand balun structure¹ with characteristic impedances Z₁ , Z₂ , Z₃ and Z₄ . The electrical length of each transmission line is λ/4 ( = 90°). The signal transmitted from the unbalance port to the two balance ports travels λ/4 and 3λ/4 wavelengths. Therefore, the output phase difference between the balance ports is 180° (3λ/4 ­ λ/4 = λ/2). ΔL is the narrow distance between the balanced output ports.
 

The modification shown in Figure 2 has an unbalanced input centerline with four identical microstrip lines of λ/4 length to obtain a bilateral balanced output. This designed balun consists of a signal input at an unbalanced port and ±90° phase difference at the bilateral balanced outputs with a phase difference of | ₂₁ ­ ₃₁ | = 180°. Experience shows that a design for 3 dB coupling will require rather narrow spaces S₁ and S₂ between the microstrip lines, which increase the complexity for its implementation.
 

The Microwave Office Program of Applied Wave Research^© was used in the design and analysis of the balun. Figure 3 shows the circuit model of the design on an FR4 substrate. The simulation results shown in Figure 4 demonstrates that the power levels |S₂₁ | and |S₃₁ | are close to ­3 dB with a variation of less than 0.3 dB from 1.9 to 3.9 GHz. The return loss |S₁₁ | is greater than 15 dB. The phase response of S₂₁ and S₃₁ with 180° difference is shown in Figure 5.

An HP 8720C network analyzer was used for the experimental measurements. The output of S₂₁ and S₃₁ are designed for ­3 dB coupling and the measured experimental results shown in Figure 6 are S₂₁ = ­3.186 dB and S31 = ­3.478 dB at 2.4 GHz. The measured results in Figure 7 show the phase difference | ₂₁ ­ ₃₁ | to be 180° ±3° from 1.9 to 3.9 GHz. Figure 8 shows a picture of the actual balun. The measured results and simulated data are in very good agreement.
 

CONCLUSION

A modification of the Marchand balun model is presented and the designed planar microstrip circuit is fairly simple and practical without complex layers. The designed balun exhibits good performance. The measured phase difference between the balance ports was within 180° ±3° from 1.9 to 3.9 GHz and the magnitudes of S₂₁ and S₃₁ are close to ­3 dB from 2.4 to 3.2 GHz. Experimental results substantiate this design with good agreement between measurement and simulation data.

ACKNOWLEDGMENTS

The authors wish to thank president William Wang, C.P. Wang, K. Chang, H.J. Chen, N. Chang and D. Chung of Global PCS Inc., Hsinchu, Taiwan, ROC, for their help and suggestions.

References

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