Basic introduction 

Balun is a kind of unbalanced transformer with the function of balance to unbalance and impedance transformation, which is used for twisted pair.


features

Balanced line and unbalanced line have different electrical characteristics, so they can not be simply connected with each other. Balanced unbalanced transformers are matched by providing impedance conversion for two different lines. Unbalanced transformer for twisted pair.

Unbalanced transformer can be used for interconnection of various types of line systems

 

effect

Unbalanced effect of balanced transmission: according to antenna theory, dipole antenna belongs to balanced antenna (bipolar antenna, dipole, biconical, Yagi), and the feeder of antenna is generally coaxial cable, which belongs to unbalanced transmission line. If it is directly connected, there will be high-frequency current flowing through the outer skin of coaxial cable (called iz in Klaus's book), and there will be radiation component when there is high-frequency current flowing through the outer skin, Even affect the polarization direction of the antenna (horizontal dipole antenna, there is current in the outer layer of the vertical RF line, which will affect the purity of horizontal polarization). Therefore, it is necessary to add a balanced unbalanced converter between the antenna and the cable to curb the current iz flowing into the outside of the cable shielding layer, that is, to cut off the high-frequency current flowing from the vibrator through the outer skin of the cable shielding layer.

Impedance transformation function: balun is essentially a transmission line transformer. According to the input impedance of the antenna, the winding method is designed to realize the impedance transformation of n ^ 2: m ^ 2. Common are 1:4, 1:9, etc.

 

Basic functions of balun:

1. Convert current or voltage from unbalanced to balanced

2. Common mode current suppression through some structures

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

 

There are many types of balun, some of which are used for impedance conversion and some are used to connect transmission lines with different impedances. Impedance conversion balun can realize impedance matching, DC isolation and matching balanced port with single ended port. The common mode choke is also a kind of balun in a sense because it can eliminate the common mode signal. Balun is used in push-pull amplifier, broadband antenna, balanced mixer, balanced frequency multiplier and modulator, phase shifter and any circuit design that needs to transmit the same amplitude and phase difference of 180 degrees on two lines.

Balun's most common use is to connect unbalanced signals to balanced transmission lines 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, balun can be used as the interface between local video, audio and digital signals and long-distance transmission lines.


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