The GSM power was unstable regardless of the power control level setting. After checking the schematics, the team found a 100 ohm resistor in the MIPI clock line that was distorting the clock waveform. This clock distortion was causing errors in decoding MIPI commands, resulting in the unstable power. Replacing the 100 ohm resistor with a 0 ohm resistor solved the issue by restoring the proper clock waveform.

1. 1

2. Symptom
 As shown below, the GSM power is unstable regardless
of PCL.
2

3. Debug
 At first, we suspected the issue is due to unstable
supply voltage and injected voltage into transceiver
and PA from external power supply, but still failed.
PATransceiver
Power
Supply
3

4. Debug
 After checking schematics, there is one 100 Ohm series
resistor in MIPI CLK, which destroys CLK waveform.
4

5. Conclusion
 Due to CLK waveform distortion, there is MIPI
command decoding error, which leads to this issue.
 After replacing the 100 Ohm resistor with 0 ohm resistor,
and the issue is solved.
5