Balun (English as balun) is a three-port device, or a broadband RF transmission line transformer that realizes the connection between a balanced transmission line circuit and an unbalanced transmission line circuit by converting a matched input to a differential output. The function of the balun is to make the system have different impedances or be compatible with differential/single-ended signaling, and is used in modern communication systems such as mobile phones and data transmission networks. Balun has the following three basic functions: 1. Convert current or voltage from unbalanced to balanced 2. Common mode current suppression through certain structures 3. Impedance conversion through certain structures (impedance ratio is not equal to 1:1) There are many types of baluns, some of which are used for impedance conversion, and some are used to connect transmission lines with different impedances. The impedance conversion balun can realize impedance matching, DC isolation, and matching of balanced ports with single-ended ports. Common-mode chokes are also a kind of balun in a sense because they can eliminate common-mode signals. The balun is used for push-pull amplifiers, broadband antennas, balanced mixers, balanced frequency multipliers and modulators, phase shifters, and any circuit design that needs to transmit on two lines with equal amplitude and 180 degrees phase difference. The most common use of baluns is to connect unbalanced signals into balanced transmission lines for long-distance transmission. Compared with single-ended signaling using coaxial cables, differential signaling using balanced transmission lines is less affected by noise and crosstalk, can use lower voltages, and is more cost-effective. Therefore, the balun can be used as an interface between local video, audio and digital signals and long-distance transmission lines. Uses of balun include: -Radio and baseband video-radar, transmitter, satellite-telephone network, wireless network modem/router Balun's basic principles The ideal S parameter of Balun is as follows: S12 = – S13 = S21 = – S31 S11 = -∞ The two outputs of the balun are equal in amplitude and opposite in phase: – in the frequency domain, this means that there is a 180° phase shift between the two outputs; – in the time domain, this means that the voltage of one balanced output is the other balanced The negative value of the output. In addition, the conductor of one of the two lines must be clearly grounded. For example, a balanced line consists of conductors with equal potential amplitudes and opposite phases. Because microstrip lines and coaxial cables use different-sized conductors, they can be described as unbalanced lines. The design purpose of the balun is to solve the problems caused by such unbalanced lines-the balun can be used in the balanced (or differential) transmission line where the current is transmitted in opposite phases and the unbalanced (or single-ended) transmission line where the return current is transmitted underground. Convert between. Inside the coaxial cable, since the electric field generated by the current inside the inner conductor and the shielding layer is limited to the space between the two, the two currents have the same amplitude and opposite phase. At the same time, the skin effect causes another current to be generated outside the shielding layer. When the current is large, the coaxial cable as the feeder can become an antenna, and the electromagnetic field whose intensity is proportional to the current is radiated outward. Because the coaxial cable has a symmetrical physical structure, and the circuits on the two internal conductors have the same amplitude and opposite phase, the internal structure itself generates very little radiation. However, some factors can destroy the current balance in the two conductors (ie, destroy the state of "equal amplitude but opposite phase"). In this case, the inside of the feeder will also generate as much current as the current outside the shield Electromagnetic radiation. This imbalance will cause distortion of the pattern, interference and loss. The key specifications when determining the type of balun for a specific application include: – Frequency coverage – Phase balance – Amplitude balance – Common mode rejection ratio – Impedance ratio/turn ratio – Insertion loss and return loss – Balanced port isolation – DC/ground isolation – Group delay flatness Balun performance index There are many types of baluns. The type of balun used in microwave radio frequency design depends on the required bandwidth, operating frequency, and the physical structure of the design. Most baluns usually contain two insulated copper wires that are twisted and wound on a magnetic or non-magnetic core. The key specifications when determining the type of balun for a specific application include: – Frequency coverage – Phase balance – Amplitude balance – Common mode rejection ratio – Impedance ratio/turns ratio – Insertion loss and return loss – Balanced port isolation – DC/ground isolation – Group delay flatness Phase balance An important standard indicator of balun is its balance, that is, two balanced outputs (one is inverted 180° output and the other is non-inverted output) and the ideal state of "equal power level and 180° phase difference" The proximity. The degree of deviation of the phase angle difference between the two outputs from 180° is called the phase unbalance of the balun. Amplitude balance This index is determined by the structure of the balun and the degree of line matching, usually in dB. Amplitude balance refers to the matching between the magnitude of the output power, and the difference between the magnitude of the two output powers is called the amplitude imbalance degree, and the unit is dB. In general, each time the amplitude balance is increased by 0.1dB or the phase balance is increased by 1°, the common mode rejection ratio (CMRR) will be increased by 0.1dB. Common mode rejection ratio (CMRR) When two identical signals with the same phase are injected into the balanced port of the balun, there may be two different results of transmission or reception. CMRR refers to the amount of attenuation that occurs when the signal is transmitted from a balanced port to an unbalanced port, and the unit is dB. CMRR is determined by the vector addition result of the two signals, and the vector addition result further depends on the amplitude balance and phase balance of the balun. Impedance ratio / turns ratio The ratio of unbalanced impedance to balanced impedance is usually expressed as 1:n. Differential impedance is the impedance between balanced signal lines and is twice the impedance of the signal line to ground. The turns ratio is a parameter of the flux-coupled balun transformer, which represents the ratio of the number of turns of the primary winding to the number of turns of the secondary winding of the transformer. The square of the turns ratio is equal to the impedance ratio. For example, when the turns ratio is 1:2, the impedance ratio is 1:4. Through the magnetic flux coupling transformer, a balun with a high impedance ratio can be designed. Insertion loss and return loss The lower the differential insertion loss and the higher the common-mode return loss, the greater the power of the inserted signal through the balun, the wider the dynamic range, and the smaller the signal distortion. In an ideal balun without isolation, the common-mode signal can be completely reflected with a return loss of 0dB, while the differential signal can pass through with a return loss of -∞. Balanced port isolation Balanced port isolation refers to the insertion loss from one balanced port to another, and the unit is dB. Since most baluns reflect even modes instead of properly terminating them with resistive loads, their balanced port isolation is not high. One exception is the 180° hybrid circuit, which outputs the even mode to a port that can be terminated with resistance. Basic types of baluns The type of balun used in a microwave radio frequency design depends on the required bandwidth, operating frequency, and the physical structure of the design. The types of baluns that can be used in differential power distribution are transformer baluns, capacitive and/or magnetic coupling transmission line baluns, hybrid coupler baluns, and such baluns can also be used for power splitters and inverters. Situation. The balun has a wide range of uses, from the conversion between single-ended signals and differential signals, to the elimination of pattern noise and signals. For balun, the most important characteristics are its power balance and phase balance. The flux coupling transformer balun is the most common type of balun, which is basically composed of a magnetic core and two different wires wound on the magnetic core. Among them, by grounding one side of the primary winding, the Equilibrium conditions, and produce equilibrium conditions on the secondary side. The ratio of the number of turns on the secondary side to the number of turns on the primary side can be arbitrarily set to produce any desired impedance ratio. The AC voltage generated on the secondary side of the magnetic flux coupling balun transformer is n times the voltage on the primary side, and the current is correspondingly 1/n of the current on the primary side, thus generating n2 times the output impedance as described above, where n is The ratio of the number of turns on the secondary side to the number of turns on the primary side. The above-mentioned wound magnetic flux coupling transformer usually has a grounded center tap in the secondary winding. This design can improve the output balance. For example, the most suitable working frequency of a magnetic flux coupling transformer is below 1 GHz. When working at a higher frequency, coupling loss often occurs. At microwave frequencies, the loss tangent of the magnetic material in the transformer is higher, which results in a larger signal loss. Therefore, a capacitively coupled transmission line balun, which is usually composed of a two-way transmission line wound on a magnetic core, such as a Guanella balun, solves the above-mentioned problems at high frequencies through low-frequency magnetic coupling and high-frequency capacitive coupling. One type of balun often used in microwave applications is the Marchand balun. The video "Various Spiral Baluns" introduces the overview of entanglement, symmetry and Marchand spiral baluns, as well as the design and simulation results of GaAs MMIC planar spiral baluns. Classic Transformer Balun A classic transformer is also called an isolation transformer, which has two independent coil windings wound on the transformer core. The core can be air (air core) or composed of magnetic neutral materials such as ceramics, magnetic conductors or soft iron. Among them, the primary winding receives the input signal, and the secondary winding outputs the converted signal. In an ideal transformer, no matter how it changes, the ratio of voltage to current is always proportional to the square of the winding turns ratio, and the power (in watts) always remains the same. Advantages: Due to the electrical isolation between the input winding and the output winding, the balun can be used to connect to a circuit where the ground voltage has a ground loop problem or an electrical incompatibility problem. Autotransformer balun (voltage balun) The autotransformer balun has one coil, or has two or more coils, and the electrical wiring of these coils is also wound on the ferrite rod core or ring core. When there is only one winding, at least one additional electrical connection or tap must be provided between the two ends of the winding. In this balun, the input current input through a pair of electrical wires functions as a primary coil and is used for magnetization of the core. Advantages: Unlike other transformer types of baluns, all ends of the autotransformer balun can ground DC current. Transmission line transformer balun (choke coil balun) This type of balun is sometimes called a current balun, which can ensure that the output currents of the two output terminals are equal, but the output voltages are not necessarily equal. The currents inside coaxial cables are equal in magnitude and opposite in phase, so the magnetic fields generated by them are equal in intensity and opposite in direction, and in most cases they can cancel each other out. When the transformer balun is combined with the transmission line transformer balun, an extremely wide working bandwidth can be realized. People often use Guanella transmission line transformer and balun combination as impedance matching transformer. Advantages: The choke balun can prevent extra current from flowing back along the transmission line through the inductive impedance. Delay line balun The delay line balun is connected to a transmission line of a specific length without any transformer, usually used in a narrow frequency range, where the length of the connected transmission line is a quarter wavelength of the target frequency in the transmission line medium multiple. Such baluns are used, for example, to switch coaxial connections to balanced antennas. Advantages: produce 180° phase shift and provide balanced input. Self-resonant balun In transformers made of physical materials, a small amount of capacitance exists between the primary winding and the secondary winding and between the turns in each winding, and these capacitances form undesirable self-capacitance or parasitic capacitance. When the self-inductance and the reactance of the self-capacitance in the balun are equal and opposite in nature, resonance occurs. When working at frequencies equal to or higher than the resonant frequency, baluns of any design type do not perform well. One consideration in balun design is to make its resonant frequency much higher than the operating frequency as much as possible. As the frequency increases, the impedance of the parasitic capacitor gradually decreases until it is equal to the impedance of the ideal inductor at the self-resonant frequency (SRF). Therefore, the above-mentioned inductance acts like an inductor whose self-resonant frequency is the critical value. Once this value is exceeded, the impedance rises sharply. Moreover, the inductor can be used as a choke coil that attenuates signals near the self-resonant frequency. Chetak Gensets,Diesel Generator,Portable Generator Silent,High Output Dc Generator Wuxi Doton Power , http://www.dotonpower.com
