Today, RF/MMIC engineers designing multi-chip modules require “circuit-level” EM simulation and modeling to meet the increased demands for higher levels of system integration. Modules providing full system-level functionality require passive support circuitry in addition to typical active components. These passive components typically include circuits such as 90-degree couplers, 180-degree couplers, in-phase couplers, filters, diplexers, and transmission line structures.
Baluns are very important support circuitry used in high-frequency circuit design. The 180-degree balun is a major component in heterojunction bipolar transistor (HBT) as well as pseudo high-electron mobility transistor (pHEMT) push-pull amplifiers, balanced mixers, balanced frequency multipliers, phase shifters, balanced modulators, dipole feeds, unbalanced to differential converters for differential signaling, and numerous other applications. Additionally, analog circuits requiring balanced inputs and outputs to reduce noise and minimize high order harmonics, and improve the dynamic range of the circuits  are also good candidates to benefit from this type of balun structure.
Derivation of a Marchand Balun
A balun, by definition, is a transformer used to connect balanced or differential transmission-line circuits to unbalanced or single-ended transmission-line circuits. Several different kinds of balun structures have been developed over the years; however, new interest in transmission-line type baluns emphasizes the need for them to be planar, compact, and more suitable for mixers and push-pull power amplifiers. In addition, several types of baluns have been used for microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs).
The most popular is the planar version of the Marchand balun because it is easy to implement and provides wide bandwidth. The Marchand balun has a documented wider bandwidth compared to other balun designs due to improved phase and amplitude balance. Looking at the development of the Marchand balun from the typical balun design illustrates why it has a superior physical layout and measurement results. Figure 1a shows the typical balun layout and Figure 1b shows the schematic of the balun.
The coaxial balun illustrated performs best when the RF currents between the center conductor and the inner conductor are in perfect balance. On the other hand, the currents on the coaxial shield can either flow on the inside or the outside of the shield. As more current flows on the outside of the shield, the amplitude and phase balance between the terminals of the balanced port degrade. Figure 2 illustrates the schematic of the coaxial balun and that the -180-degree port has characteristic impedance to ground and a corresponding resonant frequency related to the coaxial cable’s outer jacket shield.
The amplitude and phase balance of this balun varies with the impedance of the jacket to ground. The balun structure can be optimized to have improved amplitude and phase balance with the addition of a “balancing “section of coaxial cable, as shown in Figure 2a, and its corresponding schematic in Figure 2b.