Design of a two-way power splitter for wireless LAN

The power divider is a very important microwave passive device in wireless communication systems and is widely used in antenna array feeding systems, power amplifiers and wireless local area networks.

At present, the most widely used microwave power dividers are Wilkinson type power dividers, which have the advantages of simple design methods, easy implementation, and high isolation of the output port. In recent years, the research on power dividers has become more and more mature and in-depth. The method of adding short-circuit branches at the output end of the traditional Wilkinson power divider has realized a broadband power divider; the multi-section impedance transformer Wilkinson power divider structure of the reed-shaped structure significantly broadens the working bandwidth of the power divider; a new dual-frequency power divider with a planar structure; the calculation formula of the direct multi-channel output Wilkinson power divider further improves the design guidance of the power divider. However, when the operating frequency increases, the actual size of the device will be reduced. Due to the existence of isolation resistance, the circuit layout of the two output branches is limited, especially in unequal power distribution, the two output ports have strong mutual coupling, which deteriorates the overall performance of the power divider. An improved Wilkinson power divider is designed, which works in the frequency range of 2.4~2.483 5 GHz in the S band of wireless local area network, thereby increasing its practical value. ADS software was used to carry out simulation design, and physical processing and testing were carried out.

1 Power divider design

For the basic Wilkinson power divider, the input/output port characteristic impedance is Z0, and the electrical length of the two branch microstrip lines is λg 4. The basic principle and design formula of the Wilkinson power divider designed to achieve equal power division and 3 dB have been introduced in detail in reference [7], and its circuit structure diagram is shown in Figure 1. However, when the traditional Wilkinson power divider works at a higher frequency, the circuit size will be reduced, the circuit layout will be restricted, and the two output ports will be severely coupled, which will affect its performance.

In order to solve these problems, this paper improves the power divider structure shown in Figure 1 to that shown in Figure 2 by introducing microstrip transmission lines with an electrical length of 180° (λ 2) on both sides of the isolation resistor and the two output branches.

The improved Wilkinson circuit structure greatly improves the flexibility of circuit layout by introducing a transmission line with a length of λ 2. According to the transmission line theory, the matrix A of the isolation circuit part at the center frequency is:

It can be seen from matrix A that the isolation circuit part between the two output branches is still equivalent to a series resistor. The introduction of two λ/2 length transmission lines not only does not change the performance of the circuit, but also increases the distance between the microstrip lines of the two output ports, thereby reducing mutual interference.

2 Simulation and Experimental Results

Based on the above analysis and calculation, a two-way power divider for wireless LAN is designed. The center frequency is f0=2.45 GHz, the frequency range is 2.4~2.483 5 GHz, the input/output port impedance Z0=50 Ω, and the isolation resistance R=100 Ω. The F4B series microwave dielectric material board is selected, with a relative dielectric constant of &epSILon;r=2.65, a loss tangent tan δ=0.001, and a thickness of h=2 mm.

ADS simulation software was used to perform a lot of simulation optimization, and the optimal circuit size and the final processed object were obtained as shown in Figure 3. The Agilent N5230A vector network analyzer was used to measure the processed power divider. Figure 4 shows the comparison of the S parameter simulation and measured results of each port.

As shown in Figure 4, within the wireless LAN frequency band of 2.4~2.4835 GHz, the measured results show that the input port (S11<-20 dB) is well matched; the power output fluctuation is very small, S21 fluctuates by 0.2 dB, and the measured S21 at the center frequency is -3.87 dB, close to the theoretical value of -3.05 dB; the isolation between the output ports is high (S23<-25 dB), and the optimal isolation at the high-frequency end within the band exceeds 30 dB. The test results are in good agreement with the simulation results. The measured results are slightly offset to high frequencies, and the insertion loss within the band is high, which may be caused by processing and measurement errors; the performance deterioration at the high-frequency end outside the band may be due to the low accuracy of the connector and matching load.

3 Conclusion

This design is based on the traditional Wilkinson power divider theory. By introducing the λ 2 microstrip transmission line and increasing the distance between the two output ports, the flexibility of the circuit layout is improved, thereby improving the performance of the power divider. The measured results show that the power divider has good matching, power division and isolation performance in the entire design frequency band, verifying the feasibility of the scheme.