BALUN (BALanced to UNbalanced conversion)

Last updated on Mar 2, 2023

A balun, short for balanced to unbalanced, is a type of electronic transformer that converts signals between balanced and unbalanced circuits. Baluns are commonly used in radio and telecommunications applications to connect balanced transmission lines, such as twisted pair or coaxial cable, to unbalanced loads, such as antennas or radio receivers. In this article, we'll discuss the principles of balanced and unbalanced circuits, the need for a balun, the different types of baluns, and their applications.

Principles of Balanced and Unbalanced Circuits:

A balanced circuit is one in which two conductors are used to carry a signal, and the signal is transmitted as the difference between the two conductors. In other words, the signal is transmitted as the voltage between two wires. This method of transmission is also known as differential mode transmission. In a balanced circuit, the two conductors have equal and opposite impedance, and the signal is not affected by any external electromagnetic interference.

An unbalanced circuit, on the other hand, has only one conductor carrying the signal, and the signal is transmitted as the voltage between the single conductor and ground. This method of transmission is also known as single-ended mode transmission. In an unbalanced circuit, the signal is affected by external electromagnetic interference, as the ground plane acts as an antenna that can pick up unwanted signals.

The Need for a Balun:

In radio and telecommunications applications, it is common to use a balanced transmission line, such as twisted pair or coaxial cable, to carry signals from one location to another. However, most antennas and radio receivers are unbalanced, which means that they have a single input or output port. To connect a balanced transmission line to an unbalanced load, a balun is needed.

The primary purpose of a balun is to match the impedance of the balanced transmission line to the impedance of the unbalanced load. The impedance of a balanced transmission line is typically around 100 ohms, while the impedance of an unbalanced load can vary widely. A balun can also help to reduce the common-mode noise and interference that can be picked up by the unbalanced load.

Types of Baluns:

There are several types of baluns, each with its own advantages and disadvantages. The most common types of baluns are:

Voltage Balun: A voltage balun is a type of balun that uses a transformer to convert the voltage between the two conductors of a balanced transmission line to the voltage between the single conductor and ground of an unbalanced load. The transformer has a center-tapped primary winding and a single-ended secondary winding. The voltage across the primary winding is equal and opposite, while the voltage across the secondary winding is single-ended. The ratio of the turns on the primary and secondary windings determines the impedance transformation ratio.
Current Balun: A current balun is a type of balun that uses a transformer to convert the current flowing through the two conductors of a balanced transmission line to the current flowing through a single conductor of an unbalanced load. The transformer has a two-wire primary winding and a single-ended secondary winding. The current flowing through the primary winding is equal and opposite, while the current flowing through the secondary winding is single-ended. The ratio of the turns on the primary and secondary windings determines the impedance transformation ratio.
Hybrid Balun: A hybrid balun is a type of balun that combines both voltage and current balun techniques to provide a wideband impedance transformation. The hybrid balun uses a transformer to convert the voltage and current of the balanced transmission line to the voltage and current of the unbalanced load. The transformer has a four-wire primary winding and a two-wire secondary winding. The primary winding is connected in a "quad" configuration, which cancels out the common-mode noise and interference, while the secondary winding is connected in a "bifilar" configuration, which provides a wideband impedance transformation.
Transmission Line Balun: A transmission line balun is a type of balun that uses a length of transmission line to transform the impedance of a balanced transmission line to the impedance of an unbalanced load. The transmission line balun has a section of transmission line with a characteristic impedance of around 50 ohms. The length of the transmission line is chosen such that the input impedance of the balun matches the output impedance of the balanced transmission line, while the output impedance of the balun matches the input impedance of the unbalanced load.

Applications of Baluns:

Baluns are used in a variety of applications, including:

Antenna Systems: Baluns are commonly used in antenna systems to match the impedance of the antenna to the transmission line. In most cases, the antenna is unbalanced, while the transmission line is balanced. A balun is used to convert the unbalanced signal from the antenna to a balanced signal that can be transmitted over the transmission line. The balun also helps to reduce the common-mode noise and interference that can be picked up by the antenna.
Audio Systems: Baluns are used in audio systems to convert between balanced and unbalanced audio signals. Balanced audio signals are used in professional audio systems to reduce noise and interference. A balun can be used to convert a balanced audio signal to an unbalanced audio signal that can be connected to consumer-grade audio equipment.
Telecommunications Systems: Baluns are used in telecommunications systems to connect balanced transmission lines to unbalanced equipment, such as modems and routers. A balun can be used to convert the balanced signal from the transmission line to an unbalanced signal that can be processed by the equipment. The balun also helps to reduce the common-mode noise and interference that can be picked up by the equipment.

Conclusion:

In conclusion, a balun is a type of electronic transformer that converts signals between balanced and unbalanced circuits. Baluns are commonly used in radio and telecommunications applications to connect balanced transmission lines to unbalanced loads. Baluns can be classified into different types, such as voltage baluns, current baluns, hybrid baluns, and transmission line baluns, each with its own advantages and disadvantages. Baluns are used in a variety of applications, including antenna systems, audio systems, and telecommunications systems.