1. The core viewpoint of the power divider
For a detailed theoretical derivation, see Chapter 6 of the Microstrip Circuit. The core conclusions in this book are only quoted here. It is also strongly recommended to read this chapter carefully. The methodology is a magic weapon for solving many microwave circuit problems. The power splitter can be split into two odd-mode and even-mode two-port networks for analysis based on symmetry. The schematic diagram is shown in Figure 1.
(a) N-wide wide band splitter
(b) Even mode feed equivalent circuit
(c) odd-mode feed equivalent circuit
Figure 1. Theoretical analysis of the power divider
The core conclusions are:
• For a common 1 port, the odd-mode feed does not work for it. As long as the even-mode circuit is analyzed, the echo of one port and the one-port echo of the even-mode circuit are equal. The transmission line design of the power splitter thus becomes a design of a 2*Zo to Zoo impedance converter.
· The even mode feed circuit is also the working principle of the power combiner. The 2-port and 3-port inputs have the same amplitude voltage and can be transmitted to 1 port without loss. Some people often confuse S21=-3dB, according to the reciprocity of the passive circuit S12=-3dB, so there is no way to double the power. This situation occurs when there are only 2 input signals and 3 ports have no signal input.
· From the odd-mode circuit, the pure odd-mode feed public port has no power to reach. The isolation resistor acts purely as an attenuator/load, completely absorbing the power fed by the odd mode, thus achieving low standing wave of 2 and 3 ports and high isolation.
2. Simulation design of power divider
According to the previous theoretical analysis, a power divider design has actually become a design of an impedance transformer. There are many implementations of impedance transformers: stepped impedance converters/transformers/impedance converters can be implemented. The most common contact is the step impedance converter, but in fact the power divider of the transformer and impedance conversion filter structure has very good effects in many special scenarios.
The following is a detailed design process using a 2 to 18 GHz power divider. The plate is made of rogers5880 t=0.254mm. Principle and electromagnetic simulation example file download: link: https://pan.baidu.com/s/1htkfhzu Password: vwj4
1) Design steps
· Impedance converter order and each section impedance confirmation · Isolation resistance value confirmation · Separate impedance converter electromagnetic simulation · T-section electromagnetic simulation · Entire splitter combined electromagnetic simulation
2) Impedance converter order and parameter acquisition
There are many small softwares for obtaining the order and parameters of the impedance transformer. Here, the corresponding parameters are obtained by modeling and simulation in ADS. The model is shown in Figure 2. The effective dielectric constant can be obtained by the transmission line calculation tool. The initial length is the center frequency. The 1/4 wavelength, other impedance values ​​are arranged from high to low, and the impedance of each level can be obtained through simple optimization.
Figure 2. Impedance converter model and results in ADS
The actual width of each impedance can be calculated by the transmission line calculation tool (ADS comes with or polar si9000) as shown in Table 1. (plate rogers 5880 Er=2.2 t=0.254mm)
Table 1, the impedance and line width correspondence table of each section
Z1 | Z2 | Z3 | Z4 | Z5 | Z6 | Z7 | Z8 | Zo | |
impedance | 91.7 | 86.1 | 80 | 73.8 | 67.9 | 62.6 | 58.2 | 54.6 | 50 |
Line width | 0.23 | 0.27 | 0.32 | 0.37 | 0.44 | 0.51 | 0.58 | 0.64 | 0.74 |
3) Isolation resistance simulation
According to the result of the second step, a complete theoretical model of the power splitter is established in the ADS, the transmission line parameters are fixed, the isolation resistor is added, and an initial value is given arbitrarily. The isolation resistance can be conveniently obtained by a 2-port standing wave optimization. Value. The circuit optimization results are shown in Figure 3.
Figure 3. Complete functional model of the power splitter
4) Electromagnetic simulation of impedance transformer and T-section
According to the parameters calculated above, the electromagnetic simulation can be performed, and the individual is used to simulate in the sonnet. A typical power divider circuit is shown in Figure 4. Direct simulation of this circuit is due to the large simulation time of the model. Here we decompose it into two parts to optimize the simulation and reduce the resource consumption, and then perform a joint simulation verification.
Figure 4. Typical power divider circuit
The two circuits that are decomposed are an impedance transformer of 100 to 50 ohms and a T-junction of 50 to two 100 ohms.
Figure 5. Two decomposition circuits of the power splitter
According to the size of the impedance transformer calculated in step 2), the impedance transformer model of Fig. 5 is established (the bending process is performed for the reduced length, and the bending angle can be calculated by itself). The simulation results obtained without any optimization are shown in Figure 6, which shows that the parameters calculated in the ADS model are very accurate.
The simulation results of the T-section are shown in Fig. 6. Since I cut the angle at the T-section according to experience when I built the model, the simulation results of the T-section are also better. Actually, you may need to adjust the T-section cut angle when you design your own. Please feel it yourself.
Figure 6. Simulation results of impedance converter and T-section
5) Overall simulation of the power splitter model
The overall simulation of the power splitter is to combine the impedance converter and the T-section into the whole of the power splitter of Figure 4. The convenience of the netnet circuit simulation of Sonnet is introduced here. A new netlist circuit in the sonnet is shown in Figure 7, which is actually a simulation of the sub-circuits of step 4). Other large circuits can be divided into small circuits in this way and then combined with individual sub-circuits using netlist, which speeds up simulation and optimization. The comparison between the Netlist circuit simulation results and the full circuit simulation results is shown in Figure 7. The correctness of the circuit segmentation and the accuracy of the entire power divider circuit simulation process can be seen. After the resistor is finally brought in, the basic simulation of the entire power splitter is completed.
Figure 7, netlist circuit simulation and full circuit simulation results comparison
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