Balun

A balun ( balanced- unbalanced English ) is in electrical engineering and radio frequency technology, a component for converting between a balanced signal and an asymmetric signal. Especially in the high-frequency technology, the term is used balun. Baluns work in both directions, so there is the term " Asymmetrierglied " not.

Symmetric means that two be present to ground potential equal opposite phase AC voltages, such as ribbon cables and symmetric antennas. The asymmetric signal is transmitted via a coaxial cable.

Baluns often also act as an impedance transformer for power adjustment; particularly in audio engineering baluns are also used for electrical isolation.

• 2.1 High-Frequency Engineering
• 2.2 Audio Technology

Construction

Balun transformer with

At frequencies up to a few 100 MHz can be used usually an assembly of a coil with center tap, which is at ground potential. This arrangement acts as an autotransformer. From the principles of a transformer, it follows that at the balanced side is four times as large as the impedance to be connected to the unbalanced side. With separate primary and secondary windings can be achieved, other conditions except for transformation potential separation.

At low demands on the quality of it is enough occasionally to prevent standing waves of high frequency signals by a few turns of the coaxial cable or by deferring the ferrite ring cores ( standing wave trap ). Then the inner and outer conductors of the cable act like windings of a transformer with a transmission ratio of 1:1 ( current-compensated choke ). In this case, there is no impedance transformation.

Balun circuits with delay lines

At very high frequencies the losses of the delay lines are less than the inductances, therefore is used here circuits with delay line, which is usually implemented as a coaxial cable. However, these circuits operate only in a small frequency range, because the length of the line must be exactly the same for a specific multiple of the wavelength λ.

In the simplest case can be a standing wave trap achieved by a λ/4-Topfkreis. As with the circuit of current-compensated choke, the impedance is not changed.

A balun with λ/2-Umwegleitung works like this: The phase of an AC voltage at terminal A (pictured above right) is shifted along the ( copper-colored ) line by 180 °, the amount of current and voltage do not change as a result. The voltage between the left end and right end B A of this bypass line is twice as high as between one end and the shield because of the antiphase.

On the left terminal B of the baluns this phase rotated signal with the local AC voltage is connected in parallel. Therefore, the currents of the two antenna halves add up. If one calculates the apparent resistance between A and B, the result is four times the value of the coax connector.

Baluns with electronic chips

Digital signals are always unbalanced generated or processed in computers. The signals are transmitted to other computers is done but almost always symmetrical Ethernet or USB, because as particularly low interference. The considerable range of signals from zero to several 100 MHz prohibits the use of transformers, so the conversion by fast electronic circuits is performed, as shown in the adjacent picture. On the receiver side is similar to the circuit of a differential amplifier. Although the circuits such as baluns work, they are usually not referred to as such.

Applications

High Frequency Technology

In the high-frequency technique are employed for power transmission mostly unbalanced coaxial cable ( wave impedance 50, 60, or 75 Ω ) because, radiate nor absorb energy either and therefore do not cause interference. However, antennas are often symmetrical dipoles whose properties are degraded by unbalanced port ( mismatch ). A balun is therefore often part of antenna systems in HF, VHF and UHF. In open λ/2-Dipolantennen the impedance close to 75 Ω; when connected to coaxial cable with 75 Ω characteristic impedance no impedance transformation is needed and it can be used baluns that work on the principle of standing wave trap. On the other hand is at Faltdipolen, as used for example in the Yagi antennas, the impedance of 300 Ω; for coaxial antenna cable with 75 Ω characteristic impedance is a 4:1 impedance transformation is necessary ( Balun with autotransformer or λ/2-Umwegleitung ).

Baluns are also used between the symmetrical and asymmetrical ribbon cables coaxial cables. Here is an impedance transformation is usually also necessary. The characteristic impedance of the strip line, for example, 240 Ω is about four times the wave impedance of typical coaxial cable 50 ... 75 Ω, therefore, baluns can also be used with the 4:1 impedance transformation here.

Audio Technology

In audio engineering often balanced line connections (eg between guitar, microphone and mixer or amplifier ) used to suppress common mode noise. In mixer or amplifier but an unbalanced signal is often required. For conversion to a running as a transformer balun can be used with separate primary and secondary windings. This is usually referred to as a transducer. These circuits are, however, nowadays often replaced by electronic differential amplifier, since they have less distortion. Exchangers, however, cause less noise.