Design of a New Gysel Power Divider/Combiner

In microwave circuits, power dividers/combiners are very important devices, which are widely used in feeder systems, mixers and power amplifiers. Gysel power dividers/combiners were proposed by Ulrich H. Gysel in 1975. Its topology is between the branch line coupler structure and the Wilkinson structure. Like the branch line coupler, its terminal load can be connected to any position on the circuit through a transmission line of any length and with the same characteristic impedance as the load impedance. Therefore, the load can be connected to the circuit as needed, which is convenient for the use of high-power loads. At the same time, it has the same relative bandwidth as the Wilkinson power divider/combiner. In addition, the Gysel power divider/combiner can be implemented in various forms such as coaxial lines, strip lines, air plate lines and microstrip lines. However, the Gysel power divider/combiner also has some disadvantages: First, under the condition of 20% relative bandwidth, the Gysel form has better indicators such as insertion loss and standing wave than the Wilkinson form, showing better broadband characteristics. However, in the case of narrowband, when the transmission line loss is the same, the loss value of the Gysel form is about 0.12 dB, while that of the Wilkinson is 0.1 dB, and the standing wave is slightly larger. Secondly, under the principle that the return loss of the input and output ports is less than -20 dB, the relative bandwidth of the Gysel power divider/synthesizer is about 20%. In some broadband applications, the bandwidth of the Gysel power divider/synthesizer still needs to be improved. In addition, when designing the Gysel power divider/synthesizer, the impedance value of each microstrip branch is not completely determined. The resistance value of the λ0/2 microstrip line between the two isolation resistors changes with the requirements of bandwidth, isolation and other indicators, which is not conducive to design and application.

This paper improves the Gysel power divider/combiner to improve its broadband characteristics such as isolation and return loss. By improving the entire topology, the new power divider/combiner has significantly better insertion loss, return loss, isolation and other indicators than the Gysel power divider, and the impedance value of each microstrip branch is fixed, which is very convenient for design.

1 Structure and principle analysis

The structure of the traditional microstrip Gysel power divider is shown in Figure 1, which consists of four λ0/4 microstrip lines and one λ0/2 microstrip branch. The typical analysis methods for the Gysel power divider are the odd-even mode analysis method and the unit element analysis method. Generally speaking, the impedance value of each microstrip branch of the Gysel power divider/combiner can be taken as:

1.1 Odd-Even Mode Analysis

For simplicity, all impedances are normalized by the characteristic impedance Z0 of port 1. The improved Gysel power divider/combiner in normalized and symmetrical form is shown in Figure 3. When a voltage source is connected to the output port, when Vg2 and Vg3 are equal in amplitude and phase, it is an even-mode excitation. At this time, the central symmetry plane is equivalent to a magnetic wall (open circuit). When Vg2 and Vg3 are equal in amplitude and phase opposite, it is an odd-mode excitation. At this time, the central symmetry plane is equivalent to an electric wall (short circuit).

In the even-mode state, Vg2=Vg3=2V0 is taken, and its equivalent circuit is shown in Figure 4(a). From the impedance transformation of the λ0/4 microstrip, it can be seen that the port 3 is equivalent to a short circuit, so the isolation network is equivalent to an open circuit at port 2. The impedance from port 2 to port 1 is:

1.2 Comparative Analysis of Bandwidth, Isolation and Insertion Loss

The even-odd mode analysis method cannot explain the junction effect of the transmission line, and the type of transmission line junction used has a very important influence on the bandwidth performance of the entire power divider/combiner. This paper uses Agilent ADS software for simulation optimization to obtain the performance curve of the improved Gysel power divider, and compares it with the performance of the traditional Gysel power divider. The results are shown in Figures 5 to 7. Because the structure of the power divider/combiner is symmetrical, the curves of S21 and S31 are basically the same, so this paper only gives the simulation curve of S21.

As shown in Figure 5, under the principle that the return loss of the input and output ports is less than -20 dB, the bandwidth of the improved Gysel power divider/synthesizer is about 30%, which is significantly better than the traditional Gysel power divider (bandwidth is about 20%), showing good broadband characteristics. Moreover, as shown in Figure 6, within the 10% frequency band, the insertion loss of the improved Gysel power divider/synthesizer is about 0.015 dB smaller than that of the traditional Gysel power divider/synthesizer. In addition, as shown in Figure 7, the isolation of the improved Gysel power divider/synthesizer is significantly better than that of the traditional Gysel power divider/synthesizer in a wide frequency band.

2 Examples

According to the above analysis, a four-way power divider/synthesizer with a C-band bandwidth of 600 MHz was designed by adopting the multi-stage cascade implementation of microstrip circuits (see Figure 8 for the actual picture). The number of loops in the first-stage circuit isolation network is 3, mainly because the requirements for the outermost power divider of the synthesis array are the highest, while the requirements for the innermost power divider can be lower and lower. The performance test data of the four-way power divider are shown in Tables 1 and 2. From the test data, it can be seen that the loss of each port is small, the amplitude balance is good (less than 0.15 dB), the port standing wave in the working frequency band is less than 1.25, and the isolation between each distribution port is greater than 24 dB, which is consistent with the software simulation results. The large insertion loss is mainly due to the large structural size required in actual application, which leads to a long microstrip transmission line in the circuit and a large transmission loss. In actual application, the distributor is used as a power divider of the excitation stage, and the insertion loss meets the use requirements.

3 Conclusion

The working principle and topology of an improved Gysel power divider/combiner are introduced. By comparing the performance with that of the traditional Gysel power divider/combiner, it can be seen that the power divider/combiner has the advantages of wide bandwidth, low loss and high isolation. On this basis, a C-band four-way power divider/combiner is designed and manufactured, and its performance indicators meet the expected requirements.