Balun
A balun ( English bal anced- un balanced ) is in electrical and high-frequency technology , a component for conversion between a balanced line system, and an unbalanced line system. The term balun is also used for the component, especially in high-frequency technology . Baluns work both ways.
Symmetrical means that there are two alternating voltages in phase opposition to ground potential , for example in the case of ribbon cables and symmetrical antennas . The asymmetrical signal transmission takes place mainly via coaxial cables or striplines .
Often baluns also act as impedance converters for power adjustment ; Especially in audio technology , baluns are also used for potential separation .
construction
Balun with transformer
For frequencies up to a few 100 MHz, an arrangement consisting of a coil with a center tap that is at ground potential is usually used. This arrangement acts as an autotransformer . It follows from the laws of a transformer that an impedance four times as high must be connected to the symmetrical side as to the asymmetrical side. By using separate primary and secondary windings, other transformation ratios can be achieved in addition to potential separation.
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If the quality requirements are low, it is sometimes sufficient to prevent standing waves from high-frequency signals by a few turns of the coaxial cable or by sliding ferrite ring cores (standing wave barrier). Then the inner and outer conductors of the cable act like the 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
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At very high frequencies, the losses of delay lines are lower than those of inductances , which is why circuits with delay lines, which are usually designed as coaxial cables, are used here. However, these circuits only work in a small frequency range because the length of the line must be exactly a certain multiple of the wavelength λ.
In the simplest case, a standing wave barrier can be achieved using a λ / 4 cup circle. As with the circuit with a current-compensated choke, this does not change the impedance.
A balun with a λ / 2 detour line works like this: The phase position of an alternating voltage at connection A (top right in the picture) is shifted by 180 ° along the (copper-colored) line, the amounts of current and voltage do not change as a result. The voltage between the left end B and the right end A of this detour line is twice as high as between one end and the shield because of the phase opposition .
At the left connection B of the balun, this phase-shifted signal is connected in parallel with the AC voltage there. Therefore the currents of the two antenna halves add up. If you calculate the apparent resistance between A and B, you get four times the value of the coax connection.
Balun circuits with electronic components
Digital signals are always generated or processed asymmetrically in computers. The signals are almost always transmitted to other computers symmetrically via Ethernet or USB , because this means that there is particularly little interference. The considerable bandwidth of the signals from zero to a few 100 MHz prohibits the use of transformers, which is why the conversion is carried out by fast electronic circuits, as shown in the adjacent picture. On the receiver side, the circuit is similar to a differential amplifier . Although the circuits act like baluns, they are usually not called that.
Applications
High frequency technology
In high-frequency technology, asymmetrical coaxial cables (wave impedance 50, 60 or 75 Ω) are mostly used for energy transmission, because they neither emit nor absorb energy and therefore do not cause any interference. Antennas, however, are often symmetrical dipoles whose properties are impaired by an asymmetrical connection (mismatch). A balun is therefore often part of antenna systems for shortwave , VHF and UHF . With open λ / 2 dipole antennas, the impedance is close to 75 Ω; When connecting to coaxial cables with 75 Ω wave impedance, no impedance transformation is necessary and baluns that work on the principle of standing wave barrier can be used. In contrast, in the case of folded dipoles , such as those used in Yagi antennas , the impedance is approx. 300 Ω; For coaxial antenna cables with 75 Ω wave impedance, a 4: 1 impedance transformation is necessary (balun with autotransformer or λ / 2 bypass line).
Baluns are also used between the symmetrical ribbon cables and asymmetrical coaxial cables. Here, an additional impedance transformation is usually necessary. The wave impedance of the ribbon cable with 240 Ω, for example, is about four times the wave impedance of typical coaxial cables with 50 ... 75 Ω, so balun circuits with 4: 1 impedance transformation can also be used here.
Audio technology
In audio technology , symmetrical line connections (e.g. between guitar, microphone and mixer or amplifier) are often used to suppress common-mode interference . However, an asymmetrical signal is often required in the mixer or amplifier. A balun designed as a transformer with separate primary and secondary windings can be used for conversion. This is usually referred to as a transformer. However, these circuits are now often replaced by electronic differential amplifiers because they have less distortion. In contrast, transformers cause less noise.
literature
- Wolf Siebel: Antenna guide for HF reception. Outdoor antennas, indoor antennas, active antennas, additional devices. 3rd, revised edition. Siebel, Meckenheim 1987, ISBN 3-922221-23-8 .
- Herbert Zwaraber: Practical setup and testing of antenna systems. 9th, revised and expanded edition. Dr. Alfred Hüthig, Heidelberg 1989, ISBN 3-7785-1807-0 .
