Power level nrf52833 nrf21540

RE: Power level nrf52833 nrf21540

nordicuser91 — Wed, 02 Jul 2025 21:36:45 GMT

Thanks Jennifer

RE: Power level nrf52833 nrf21540

Jennifer Gibbs — Wed, 02 Jul 2025 21:26:21 GMT

Hello Purvesh,

As Bendik noted in his initial response:

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

The 5dBm is the absolute maximum and therefore I wouldn't suggest going past +4dBm out of the SoC.

Thank you,

Jennifer Gibbs

RE: Power level nrf52833 nrf21540

nordicuser91 — Wed, 02 Jul 2025 00:09:25 GMT

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.

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

Problem:

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.
I expected a linear increase, since theoretically:
- 0 dBm + 10 dB (FEM PA gain) ≈ 10 dBm
- +4 dBm + 10 dB ≈ 14 dBm
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

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.

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?

RE: Power level nrf52833 nrf21540

nordicuser91 — Tue, 01 Jul 2025 22:42:23 GMT

Thanks for information.

RE: Power level nrf52833 nrf21540

Bendik Heiskel — Wed, 04 Jun 2025 12:58:41 GMT

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.

RE: Power level nrf52833 nrf21540

nordicuser91 — Tue, 03 Jun 2025 15:50:22 GMT

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?

RE: Power level nrf52833 nrf21540

Bendik Heiskel — Tue, 03 Jun 2025 14:11:19 GMT

Hi,

[quote user=""]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?[/quote]

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

[quote user=""]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?[/quote]

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.

[quote user=""]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?[/quote]

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