Power level nrf52833 nrf21540

[nordicuser91]

Hi,

I’m currently working with the nRF52833 together with the nRF21540 Front-End Module (FEM). I have configured the FEM to provide a gain of 10 dBm. I have a few questions regarding the appropriate TX power settings:

1. If I set the nRF52833 output power to 0 dBm and the nRF21540 gain to 10 dBm, will the total output power be 10 dBm?

2. If I increase the nRF52833 output power to 4 dBm while the FEM remains at 10 dBm gain, will that result in a 14 dBm total output, or is the FEM capped at 10 dBm total output?

3. Would setting the nRF52833 to 4 dBm risk damaging the FEM, or is it safe to operate under this configuration? Is there anything specific I should consider both in terms of hardware and firmware to have the best performance?

Thanks

Regards

[Bendik Heiskel]

Hi,

If I set the nRF52833 output power to 0 dBm and the nRF21540 gain to 10 dBm, will the total output power be 10 dBm?

Yes, this combination will result in total output power of the 10dBm.

If I increase the nRF52833 output power to 4 dBm while the FEM remains at 10 dBm gain, will that result in a 14 dBm total output, or is the FEM capped at 10 dBm total output?

The output power of the FEM will be 14dBm with 4dBm input. The amplifier in the nRF21540 saturates at 21.5dBm output power, so for 10dB gain setting the total output power will always be SoC output power + 10dB, as long as the SoC output power is lower than 5dBm.

Would setting the nRF52833 to 4 dBm risk damaging the FEM, or is it safe to operate under this configuration? Is there anything specific I should consider both in terms of hardware and firmware to have the best performance?

4dBm out from the nRF52833 is OK, as the maximum input power tolerated at the TRX pin is 5dBm.

You don't really have to worry about setting the FEM gain and the SoC output power manually in the FW if using the nRF Connect SDK, as the nRF21540 driver will automatically configure both the FEM gain and the SoC power to best match the configured total output power. The driver will make sure that the actual output power is not higher than the requested power, so if for example a total output power of 11dBm is configured, the actual output power will be 10dBm, since the nRF52833 can't transmit at 1dBm output power.

More info on using the nRF21540 in the nRF Connect SDK can be found here:
https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/app_dev/device_guides/fem/index.html

 

Best regards,

Bendik

[nordicuser91]

Thanks for the information Bendik, I am currently using nrf52 SDK 17.x to work on this project. Can you please point me to the library for the same using this SDK?

[Bendik Heiskel]

The nRF5 SDK does not have full support for the nRF21540, it can not be used with a SoftDevice, so it does not support BLE. Only the nRF Connect SDK have fuill support for nRF21540.

[nordicuser91]

Thanks for information.

[nordicuser91]

I'm currently working with the nRF21540 and a custom driver. Only the PA (Power Amplifier) functionality is used; LNA is not active. My goal is simply to boost the TX signal from the nRF52833 during BLE transmissions.

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Setup:

• SoC: nRF52833

• FEM: nRF21540 (PA only, LNA not used)

• Connection: nRF52833 RF pin → nRF21540 → Antenna

• Behavior:

  • TX: FEM is enabled and configured only during BLE transmission

  • RX: FEM is bypassed entirely

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Problem:

1. When I configure the nRF52833 TX power to +8 dBm or +9 dBm, the measured output power saturates (i.e., no increase is observed), even though the FEM is ON.

2. I expected a linear increase, since theoretically:

   • 0 dBm + 10 dB (FEM PA gain) ≈ 10 dBm

   • +4 dBm + 10 dB ≈ 14 dBm

3. However, in my case:

   • +4 dBm + 10 dB gives ~5 dBm output (expected, minus system losses), measured: 

   • +8 dBm + 10 dB also gives ~5 dBm output — no additional gain

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My Understanding:

• I assume some drop in output is expected due to impedance mismatch or matching network loss, but the fact that +4 dBm and +8 dBm input produce nearly identical output suggests something else may be limiting the output.

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Questions:

• Is the nRF21540 PA saturating at high input levels?

• Is there a recommended maximum input level for optimal FEM performance?

• Could this behavior be due to:

  • Internal PA compression or limiting?

  • PCB layout or impedance mismatches becoming more severe at higher drive levels?

• Should I limit the nRF52833 output to ~0 to +4 dBm and let the PA do the rest?