New Isolation Circuits of Compact Impedance-Transforming 3-dB Baluns for Theoretically Perfect Isolation and Matching

Dept. of Electr. Eng., Univ. of California at Los Angeles, Los Angeles, CA, USA
IEEE Transactions on Microwave Theory and Techniques (Impact Factor: 2.24). 01/2011; 58(12):3892 - 3902. DOI: 10.1109/TMTT.2010.2087348
Source: IEEE Xplore


New isolation circuits and design equations of compact impedance-transforming 3-dB baluns are suggested for theoretically perfect isolation and perfect matching at all ports. Any balun consists of two impedance transformers, being 180° out of phase, and an isolation circuit. For compactness, the impedance transformers need to be reduced and the isolation circuits should depend on phase delay of the compact impedance transformers. The compact balun with -90° phase delay of the compact impedance transformer is called the compact II-type balun and the one with non -90° phase delay is called the constant voltage-standing-wave-ratio type transmission-line impedance transformer (CVT) balun. Three isolation circuits are derived for the compact II-type baluns and five for the CVT baluns. Using the isolation circuits derived, the two types of compact baluns are fabricated and measured at a design center frequency of 1.5 GHz. The measured results have good agreement with prediction, showing power divisions of -2.9 and -3.3 dB (- 2.8 and -3.25 dB ), phase difference between two output signals of 181.8°(178.5°) , matching performance of -21.8, -31.8, and -33.8 dB ( -24, -22, and - 28 dB), and isolation of better than 40 dB (27 dB) for the compact II-type (CVT) balun.

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    • "where are characteristic admittances of open stubs, and are electrical lengths of transmission-line sections and open stubs in Fig. 6. The -type of the equivalent circuit with have been used for various applications [24], [26], [29], [33], but that with greater than 2 has not been discussed yet. For the use of the equivalent circuits , the characteristic impedances of and should be realizable. "
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    ABSTRACT: New design formulas for impedance-transforming 3-dB Marchand baluns are proposed. They are about the even- and odd-mode impedances of the coupled transmission-line sections of the Marchand baluns and determined by coupling coefficients together with termination impedances. The particular property proposed in this paper is to choose the coupling coefficient arbitrarily, resulting in infinite sets of design formulas available. This is quite different from the conventional design approach in which only one coupling coefficient is possible. For the perfect isolation of the Marchand balun, an isolation circuit (IC) is needed, being composed of two 90° transmission-line sections and resistance(s). Sufficient area to build such a long IC is, however, inherently not available. For this, ways to reduce the IC size are also suggested. To validate them, a microstrip Marchand balun terminated in 130 and 70 Ω is designed at a design center frequency of 1.5 GHz and tested. The measured results are in good agreement with prediction, showing that power divisions are 3.57 and 3.262 dB, return losses at all ports are better than 21 dB, and the isolation is better than 20 dB around the design center frequency. The measured phase difference between two balanced signals is 180°±2° in about 50% bandwidth.
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