What is Balun Transformers?
A balun, short for “balance-to-unbalance,” is a specialized class of microwave passive components designed to convert the fundamental guided mode from a balanced configuration to an unbalanced one, or vice versa. This conversion often involves transforming the characteristic impedance along with the line-to-line modal conversion. Baluns are employed in various applications to achieve desired signal transformations.
Typically, a balun structure involves two distinct transmission line geometries serving as the input and output of the circuit. While early versions were proposed for coaxial lines and nonplanar structures, modern planar baluns, implemented through two-sided and uniplanar Microwave Integrated Circuit (MIC) technologies, have gained significant attention.
Baluns can be broadly categorized into two types, as outlined by Trifunovic and Jokanovic in 1994:
Marchand Baluns (Band-Pass Networks)
In Marchand baluns, the input and output lines are orthogonally crossed over, and two ports are terminated with either a short or open end. This configuration transforms the effects of an open or short at the junction using quarter-wavelength line transformers. While Marchand baluns have a simple geometry, they tend to exhibit a narrow operating bandwidth, behaving like a band-pass network.
Double-Y Baluns (All-Pass Networks, Ideally)
Double-Y baluns are based on a six-port double-Y junction with three balanced and three unbalanced lines placed alternately around the center of the junction. Input and output lines are typically aligned along the same axes, while the other four branch lines are either short- or open-circuited, performing in-phase and out-of-phase signal dividing/combining procedures. This design is akin to the well-known low-frequency electrical bridge transformer. Double-Y baluns are often favored for broadband designs.