Design and characterization of multilayer spiral transmission-line baluns

• Published in: IEEE transactions on microwave theory and techniques 1999-09, Vol.47 (9), p.1841-1847
• Main Authors: Yoon, Y.J., Yicheng Lu, Frye, R.C., Lau, M.Y., Smith, P.R., Ahlquist, L., Kossives, D.P.
• Format: Article
• Language: English
• Subjects: Conductors; Couplings; Frequency; Glass; Impedance matching; Insertion loss; Nonhomogeneous media; Silicon; Spirals; Transmission lines
• ISSN: 0018-9480; 1557-9670
• DOI: 10.1109/22.788521

We discuss the design of coupled spiral transmission-line baluns modeled after the Marchand type. The balun structure consists of a pair of coupled spiral conductors vertically offset across intervening polyimide layers. The baluns are fabricated on various substrates (glass and high- and low-resistivity silicon). The characteristics such as return loss, insertion loss, and output signal imbalance are measured. The center frequencies of 3-dB bandwidths (BWs), primarily determined by their conductor lengths, range from 1.2 to 3.5 GHz. The 3-dB BW normalized by the center frequency is /spl sim/1.48 in all cases. We observe an optimum BW for better performance. Return losses at the center frequencies range from 13 to 18 dB. Amplitude imbalance distributes in the range of 0.3-1.0 dB, depending on the sizes of devices and substrates. The minimum insertion loss is 0.55 dB for the balun on a glass substrate with 100-/spl mu/m-wide conductors. The devices fabricated on glass and high resistivity (>4000 n cm) silicon show remarkably similar behaviors despite the large difference in dielectric constant. This technique is applicable to monolithic microwave integrated circuits.