Design and characterization of multilayer spiral transmission-line baluns

We discuss the design of coupled spiral transmission line baluns modeled af ter the Marchand type. The balun structure consists of a pair of coupled sp iral conductors vertically offset across intervening polyimide layers, The baluns are fabricated on various substrates (glass and high- and low-resist ivity silicon). The characteristics such as return loss, insertion loss, an d output signal imbalance are measured. The center frequencies of 3-dB band widths (BW's), primarily determined by their conductor lengths, range from 1.2 to 3.5 GHz, The 3-dB BW normalized by the center frequency is similar t o 1.48 in all cases. We observe an optimum BW for better performance. Retur n losses at the center frequencies range from 13 to 18 dB, Amplitude imbala nce distributes in the range of 0.3-1.0 dB, depending on the sizes of devic es and substrates, The minimum insertion loss is 0.55 dB for the balun on a glass substrate with 100-mu m-wide conductors. The devices fabricated on g lass and high resistivity (>4000 Omega.cm) silicon show remarkably similar behaviors despite the large difference in dielectric constant. This techniq ue is applicable to monolithic microwave integrated circuits.