Balun

Common-mode chokes are also used as baluns and work by eliminating, rather than rejecting, common mode signals.

The advantage of transformer-type over other types of balun is that the electrically separate windings for input and output allow these baluns to connect circuits whose ground-level voltages are subject to ground loops or are otherwise electrically incompatible; for that reason they are often called isolation transformers.

The core that they are wound on may either be empty (air core) or, equivalently, a magnetically neutral material like a porcelain support, or it may be a material which is good magnetic conductor like ferrite in modern high-frequency (HF) baluns, or soft iron as in the early days of telegraphy.

The electrical signal in the primary coil is converted into a magnetic field in the transformer's core.

The current sent into the balun through one pair of connections acts as if it were a primary coil, and magnetizes the entire core.

[3] Transmission line transformers commonly use small ferrite cores in toroidal rings or two-hole, binocular, shapes.

[5] Transmission line transformer baluns with a high frequency transmission line such as a wirewound pair are able to operate at significantly higher frequencies (to 8 GHz and above) by combining capacitive coupling with magnetic coupling typical of classic magnetic transformers.

[6] This basic circuit has been widely adapted and modified for use in planar structures (such as MMICs and RFICs) at frequencies up to at least 100 GHz.

(Straw 2005, 25-26) A balun's function is generally to achieve compatibility between systems, and as such, finds extensive application in modern communications, particularly in realising frequency conversion mixers to make cellular phone and data transmission networks possible.

This typically is needed when a balanced antenna (for instance, a dipole) is fed with coax; without a balun, the shield of the coax could couple with one side of the dipole, inducing common mode current, and becoming part of the antenna and unintentionally radiating.

Many security cameras now have both a balanced unshielded twisted pair (UTP) output and an unbalanced coaxial one via an internal balun.

Runs over 130 m (400 ft) may lose quality because of attenuation and variations in the arrival time of each signal.

[citation needed] In audio applications, baluns serve multiple purposes: they can convert between high-impedance unbalanced and low impedance balanced lines.

The common-mode rejection of interference characteristic of balanced mains power, eliminates a wide range of noise coming from the wall plug, e.g. mains-borne interference from air conditioner/furnace/refrigerator motors, switching noise produced by fluorescent lighting and dimmer switches, digital noise from personal computers, and radio frequency signals picked up by the power lines/cords acting as antennae.

The device on the left would normally be used to connect a high impedance source, such as a guitar, into a balanced microphone input, serving as a passive DI unit.

The one in the center is for connecting a low impedance balanced source, such as a microphone, into a guitar amplifier.

Pair of AC&E 120 Ω twisted pair (Krone IDC ) to 75 Ω coaxial cable balun transformers. Actual length is about 3 cm.
Isolation transformer
Circuit diagram of an autotransformer balun using three taps on a single winding on a ferrite rod.
Picture a balun of the same design, wound on ferrite toroid. Notice that the black and red winding wires are joined at the threaded connector.
Homemade 1:1 balun using a toroidal core and coaxial cable. This simple RF choke works as a balun by preventing signals passing along the outside of the braid. Such a device can be used to cure television interference by acting as a braid-breaker .
A 75-to-300- Ω balun built into the antenna plug.
Three audio transformers ; two of them baluns.