Design and Characterization of Multilayer Spiral Transmission Line Baluns

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 different substrates (glass, 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, primarily determined by their conductor lengths, range from 1.2 to 3.5 GHz. The 3-dB bandwidth normalized by the center frequency is ~ 1.48 in all cases. We observe an optimum bandwidth for better performance. Return losses at the center frequencies range from 13 to 18 dB. Amplitude imbalance distributes in the range of 0.3 to 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 micron wide conductors. Devices fabricated on glass and high resistivity silicon (>4000 OMEGA cm) show remarkable similar behaviors despite the large difference in dielectric constant, while the devices fabricated on low resistivity (~20 OMEGA cm) silicon do not exhibit the baluns characteristics.