Classification and Principle of Power Divider in RF System

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What is a power divider?

A power divider is a device that divides the energy of one input signal into two or multiple channels and outputs equal or unequal energy. It can also combine the energy of multiple signals into one output. At this time, it can also be called combiner. A certain degree of isolation should be guaranteed between the output ports of a power divider. The power divider is usually divided into one-two (one input and two outputs), one-third (one input and three outputs) and so on according to the output. The main technical parameters of the power divider are power loss (including insertion loss, distribution loss and reflection loss), voltage standing wave ratio of each port, isolation between power distribution ports, amplitude balance, phase balance, power capacity and frequency band width, etc.

The power divider is divided into two categories in structure

(1) Passive power divider, its main features are: stable operation, simple structure, basically no noise; and its main disadvantage is that the access loss is too large.

(2) The active power divider is composed of an amplifier. Its main features are: gain and high isolation, while its main disadvantage is noise, relatively complex structure, and relatively poor working stability. The output port of the power divider has two power points, three power points, four power points, six power points, eight power points, and twelve power points.

The following are analysis and comparison of several common microstrip power dividers.

Microstrip Branch Line Directional Coupler

The structure of the microstrip branch line directional coupler is shown in Figure 1. It consists of two parallel conduction bands. The coupling is realized through the two branch conduction bands. The length and interval of the branch conduction bands are both quarter-on-line wavelengths. . Ideally, there is no reflection at the input of port 1, the input power is output by ports 2 and 3, and there is no output at port 4, that is, ports 1 and 4 are isolated from each other. It can be calculated from the odd-even mode analysis method in microwave theory that for the case of equal power division, the characteristic impedance of the branch conduction band is the same as that of the input and output lines, while the characteristic impedance of the parallel conduction band is 1/ of the input and output lines. S12 and S13 have the same characteristic impedance, phase difference of π/2. The microstrip branch line bridge is mainly used as a microstrip balanced mixer. Since the ports 1 and 4 are isolated from each other, the local oscillator and the signal do not affect each other. At the same time, due to the planar characteristics of the microstrip line, the mixing crystal is easily connected to the port. On the other hand, the circuit structure is simple and compact.

Microstrip Branch Line Directional Coupler

Wilkinson Power Divider

The structure of the Wilkinson power divider is shown in Figure 2, and the characteristic impedance of its input line and output line are both. For the case of power equalization, the characteristic impedance of the branch line between the input and output ports , the line length is a quarter of the line wavelength, and a resistor R is connected across the end of the branch line, and its value is 2. It can be proved by microwave theory that when ports 2 and 3 are connected to matching loads, the input of port 1 has no reflection, and vice versa for ports 2 and 3. The power input from port 1 is divided equally between ports 2 and 3, and ports 2 and 3 are isolated from each other.

Wilkinson Power Divider

Two-wire divider

The structure of the two-wire splitter is very simple, and it can flexibly adjust the characteristic impedance of the branch line according to the given input impedance to achieve good matching, so it has been widely used in the design of the feeding network of the antenna, but its disadvantage is that there is no good isolation between the outputs.

Detailed explanation of the principle of the power divider

The power divider is a 3-port network with zero phase shift between its output ports. This three-port device is reversible, and it can be applied both in the form of power splitting and power combining. The input power of the signal input terminal is P1, and the output powers of the other two output terminals are P2 and P3 respectively. Theoretically, according to the law of conservation of energy, we can know that P1=P2+P3.

If P2=P3 and the relationship between the three-port network power is expressed in milliwatt decibels (dBm), it can be written as: P2 (dBm)=P3 (dBm)=Pin (dBm)-3dB. Of course, P2 does not have to be equal to P3, but the case of equality is most often used in practical circuits. Therefore, power dividers can be roughly divided into two types: equal parts (P2=P3) and proportional types. Its design principle is explained in sequence as follows: The following figure is the schematic diagram of the microstrip three-port power divider, which is developed on the basis of the microstrip T-joint, and its structure is relatively simple. The signal is input from port "1" (the characteristic impedance of the connected transmission line is Z0), and the two branch microstrip lines with characteristic impedance of Z02 and Z03 are respectively output from ports "2" and "3", and the load resistances are R2 and R3 respectively. There is no coupling between the two branches, and their electrical lengths at the center frequency are θ=pi/2.

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