With the advancement of 4G/5G mobile communication technology, the demand for antenna-compatible multi-band systems has significantly increased. In the design process, it is essential to test the standing wave ratio and insertion loss of the antenna. However, using a network analyzer for manual plugging and unplugging is not only time-consuming but also prone to repeated insertions, which can cause poor contact performance and make it difficult to verify the correctness of the design. Therefore, an automatic testing method is necessary to replace manual wiring tests and expand the network test ports (as shown in figure 2), connecting them directly to the antenna test port. However, during the design process, various challenges may arise, and the following step-by-step analysis will help address these issues.
**Problem Phenomenon**
During the antenna testing phase, it was observed that one channel had an abnormally high standing wave ratio and insertion loss. The maximum standing wave reached 3.1, with a difference of 20 dB. Upon inspection, it was determined that the issue stemmed from excessive standing wave and insertion loss at one of the PCB ports.
By comparing the results, it was found that when -45° port was used as input and S11 port as output, the standing wave and insertion loss were within normal limits, with a maximum standing wave of 1.53 and insertion loss of -7 dB. However, when +45° port was used as input and S22 as output (as shown in Figure 1), the maximum standing wave was 3.2, and the insertion loss reached 20 dB. Both circuits had identical structures, except for the length of the transmission line. After analysis, it was concluded that the high standing wave and insertion loss were due to impedance mismatch in the circuit. The next step is to resolve this issue.
**Figure 1: Test Port PCB Diagram**
**Figure 2: Test Port Schematic**
**Single-Port Solution**
1. Option 2: Replace the antenna switching IC. Currently, only the HMC595E meets the requirements (though a PIN diode could also be used).
2. Option 3: Modify the circuit design to match the transmission line. This option is feasible and practical.
**Multi-Port Testing**
The purpose of this test is to identify and solve the issue where ICs interact with each other, causing degradation in standing wave and insertion loss.
Test 1: +45° IN, S22 OUT, other ports left floating;
Level: U27_A low, U27_B high, U28_A high, U27_B low.
Test 2: +45° IN, S22 OUT, other ports left floating;
Level: U27_A high, U27_B low, U28_A high, U27_B low.
Test 3: -45° IN, S11 OUT, other ports left floating;
Level: U27_A high, U27_B low, U28_A low, U27_B high.
Test 4: +45° IN, S11 OUT, other ports left floating;
Level: U27_A low, U27_B high, U28_A high, U27_B low.
In Test 2, the impact of U27 on the standing wave and insertion loss of the U28 path was tested. According to theoretical analysis, when U27_A is high and U27_B is low, the U27_RF1 pin is disconnected, while U27_RF2 is connected. Similarly, when U28_A is high and U28_B is low, U28_RF1 is disconnected, and U28_RF2 is connected. This configuration is shown in Figure 4.
**Figure 4: HMC595E Pin Diagram and Truth Table**
When the signal is input through the +45° port, it passes through U28 and S22, while no signal flows through U27. However, during testing, it was found that after soldering U27, the S22 port's input standing wave slightly deteriorated, increasing from 1.7 to 2.1. As shown in Figures 5 and 6.
**Figure 5: No-Weld U27, +45 Input S22 Output Channel Standing Wave and Insertion Loss**
**Figure 6: Welding U27, +45 Input S22 Output Channel Standing Wave and Insertion Loss**
To address the impedance mismatch, the circuit was modified. Through careful debugging, the final circuit design is as follows:
**Figure 7: Final Debug Circuit**
In Figure 7, R10 = 5.1 Ω, R11 = R12 = 810 Ω, achieving a 50 Ω impedance match. The specific calculation can be done using network attenuation software or mathematical formulas. A screenshot of the software interface is shown below.
**Figure 8: Network Attenuation Calculation Software Interface**
The revised PCB layout is as follows:
**Figure 9: PCB Diagram After Debugging**
**Partial Standing Wave Test Data:**
**Table 1: System Standing Wave Part Test Data**
| | Board Port | Antenna | System |
|-------|------------|---------|--------|
| S22 | 1.54 | 1.51 | 1.78 |
| S11 | 1.55 | 1.44 | 1.58 |
**Summary:**
This approach temporarily resolves the standing wave and insertion loss issues, but it does not allow for precise determination of circuit parameters or PCB layout settings, which can hinder efficiency. In the next article, we will explore the root causes of the problem in greater detail.
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