When the customer conducted normal TX power transmission tests using the VC5755-31 on the BCM67XX platform, they encountered EVM degradation in HE80 mode during the testing process. The worst EVM value was significantly below the standard of -35dBm. Next, we will explain how to resolve this issue.
Based on the data collected from the instrument, under the HE-SU_80M bandwidth and with similar Power values, the EVM difference between the two reached approximately 13dB.
Figure 1, referencing the test report failure diagram.
Figure 2, referencing the test report approval diagram.
After multiple rounds of testing and verification, it was found that the deterioration of the EVM is an inevitable phenomenon. First, we adjusted and optimized the impedance output by the FEM, and the load position has met expectations, but the issue still persists.
Next, we need to investigate the EVM quality output from the main platform SOC. At this point, it was discovered that the EVM deviation of the FEM input did not meet the standard value.
Figure 3, Main Chip Output Test Diagram
4. After confirming the main platform's NVRAM configuration with the customer, it was discovered that one configuration with the DPD function might be related to the FEM. With the customer's consent, the software configuration was modified to verify the issue. Subsequently, the DPD function in the NVRAM configuration was disabled for testing, and the EVM returned to normal. The issue has been resolved.
1Q: What is the EVM standard for WiFi7?
The requirement for 1A: 11be MCS13 is ≤-38dBm.
2Q: Is there a reference document for RF load debugging?
2A: The manufacturer will provide the optimal load position of the chip as a reference.
3Q: Will there be any differences when using domestically produced comprehensive testers for WiFi testing?
3A: The difference is minimal, and the data can all be used as a reference.
4Q: Can harmonic spurious emissions be tested simultaneously?
4A: The requirements for harmonics and spurious signals are different and need to be switched and modified.
5Q: What should be done if the spectrum template exceeds the standard?
5A: First, optimize the filters on the Load and RF link.