Principle of Balun Transformer

Principle of Balun Transformer

Summary：

Balun is a three-port device, or a broadband RF transmission line Voltage-Switching device that realizes the connection between balanced transmission line and UN balanced transmission line circuit by converting matched input into differential output. Device. Barron's function is to make the system different impedance or compatible with differential/single-ended signaling, and used in modern communication systems such as mobile phones and data transmission networks.

Function：

Baron has the following three basic functions:

1. Transform current or voltage from unbalanced to balanced

2. Common-mode current suppression by some constructions

3. Impedance conversion through some structures (impedance ratio is not equal to 1:1)

Balun is divided into many types, some of which are used for impedance conversion, and some of which are used to connect transmission lines with different impedance. Impedance conversion balun can realize impedance matching, DC isolation and matching the balance port with the single port. 
Common-mode choke is also a kind of Baron in a sense because it can eliminate common-mode signals. Barron is used in push-pull amplifiers, broadband antennas, balanced mixers, balanced multipliers and modulators, phase shifters, and any circuit design requiring equal transmission amplitude and 180 degree phase difference between two lines.

purpose:

Barron's most common use is to connect unbalanced signals to a balanced transmission line for long-distance transmission. Compared with single-ended signaling using coaxial cable, differential signaling using balanced transmission line is less affected by noise and crosstalk, can use lower voltage and is more cost-effective. Therefore, Barron can be used as an interface between local video, audio and digital signals and long-distance transmission lines. Balun's uses include:

- Radio and Baseband Video

- Radar, transmitter, satellite

Telephone network, wireless network modem/router

Principle

Baron's ideal S parameters are as follows:

S12 = S13 = S21 = S31

S11 = - Infinity

Balun's two outputs are equal in magnitude and opposite in phase:

In the frequency domain, this indicates a phase shift of 180 degrees between the two outputs.

In the time domain, this means that the voltage of one balanced output is negative to that of another balanced output.

In addition, one conductor of the two lines must be clearly grounded.

For example, a balancing circuit consists of conductors with equal potential amplitude and opposite phase. Because microstrip and coaxial cables have different conductors of different sizes, they can be called unbalanced lines. Balun's design aims to solve the problem caused by such unbalanced lines - Balun can switch between the balanced (or differential) transmission line in which the current is transmitted in opposite phase and the unbalanced (or single-ended) transmission line in which the return current is transmitted underground.

In coaxial cable, the electric field generated by the current inside the inner conductor and shielding layer is confined to the space between them, so the two currents have the same amplitude and opposite phase. At the same time, skin effect causes another current to be generated outside the shielding layer. When the current is large, the coaxial cable as the feeder can be made an antenna, and the radiation intensity is proportional to the current. Because the coaxial cable has symmetrical physical structure, and the circuit on its inner two conductors has the same amplitude and opposite phase, the radiation generated by its inner structure itself is very small. However, some factors can destroy the current balance in the two conductors (i.e., destroy the state of "equal amplitude, opposite phase"). In this case, the inner part of the feeder will also produce large electromagnetic radiation just like the outer part of the shield. This imbalance will lead to distortion, interference and loss of the pattern.

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