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 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 (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 to 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-mu m-wide conductors. The devices fabricated on glass and high resistivity (>4000 Omega.cm) silicon show remarkably similar behaviors despite the large difference in dielectric constant. This technique is applicable to monolithic microwave integrated circuits.