nrf21540 gain always 20dB

Question

Hello, 
 
 We are testing the radio_test sample on a nrf21540dk board with ncs-2.3. We want to change the fem tx_gain but we always get the same RSSI value on the nrf connect RSSI VIEWER application. 
 
 It's like we always operate with a 20dB gain. 
 
 We added the following configuration to the radio_test sample : 
 
 CONFIG_FEM=y
CONFIG_MPSL=y 
CONFIG_MPSL_FEM=y
CONFIG_MPSL_FEM_NRF21540_GPIO_SPI=y
CONFIG_MPSL_FEM_ONLY=y
CONFIG_MPSL_FEM_NRF21540_RUNTIME_PA_GAIN_CONTROL=y
CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB=10

CONFIG_RADIO_TEST_POWER_CONTROL_AUTOMATIC=n 
 
 Then we used the following commands : 
 
 start_channel 1
end_channel 80
time_on_channel 50
fem tx_gain 10
output_power pos0dBm
start_tx_sweep 
 
 Are we missing something ?

Bendik Heiskel · Accepted Answer

The nRF52840 have a absolute maximum input power of 10dBm, meaning it can be damaged by input powers of 10dBm or higher. The receiver will be saturated before this, so you won't really be able to measure a difference in RSSI between 20 and 10dBm output power from the nRF21540DK. 
 Do you have a spectrum analyzer available to measure the output power? 
 Here is the radio test FW nRF21540 I used for the measurement I showed earlier: nRF21540dk_radio_test_NCS_v2.3.0.hex

nrf21540 gain always 20dB

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