FEM issue

RE: FEM issue

stak — Thu, 03 Sep 2026 14:13:56 GMT

Thank you for the update, and my apologies for the late reply.

After deep diving into the RFX2401C datasheet, no clear improvement after changing ctx-settle-time-us and crx-settle-time-us to 23 µs / 5 µs seems to be not helpfull for your type of measurements, however it should be still valid or corrected by you for fine tuning in such a setup: RFX2401C + SoC radio rampup.

I think the more likely issue is tx-gain-db configuration and how MPSL splits TX power on simple GPIO FEM.

For simple GPIO FEM, tx-gain-db must match real PA gain at your operating point. SKY66112 defaults to 22 dB. RFX2401C is not in Nordic’s supported FEM table - gain must be validated on your hardware.

For antenna power requests, the stack calls mpsl_fem_tx_power_split, returning mpsl_tx_power_split_t with RADIO.TXPOWER and FEM control via mpsl_fem_pa_power_control_set. Simple GPIO FEM has no built-in power model - MPSL uses fixed tx-gain-db from devicetree: radio_tx_power = requested − tx_gain_db

If you need other tx power map you can use custom compensation via mpsl_fem_power_model_set / fetch api - it is not provided for third-party FEMs.

Real antenna power ≈ SoC tx power + actual PA gain - mind that outcome is not a simple arithmetic calculation if gain is wrong or SoC drive is outside the PA linear region of FEM.

Example:

FEM config tx-gain-db = 25, actual PA gain ≈ 22 dB:

*No FEM set to max SoC +8 dBm (in nRF54L15 case).
**RFX2401C saturated output max ~+22 dBm - stack reports +25 dBm but PA cannot deliver it.

When requested antenna power equals tx-gain-db, SoC tx power is set to 0 dBm - a sensible FEM operating point (moderate/default PA input).

With tx-gain-db = 22 and request +22 dBm, SoC = 0 dBm, reported +22 dBm, real antenna ≈ +22 dBm if gain is correct - aligned with RFX2401C max saturated output.

Our recommendation:

• Operate at valid FEM power points — choose requested antenna power so SoC tx power stays in the PA’s linear region. When requested power = tx-gain-db, SoC = 0 dBm (e.g. tx-gain-db = 22 → request +22 dBm for RFX2401C max). Avoid high requested power with overstated gain that pushes SoC deep below 0 dBm (+11 dBm with tx-gain-db = 25 → −14 dBm SoC).

• Or apply a custom power model — if target power cannot align with a valid operating point, implement mpsl_fem_power_model_set with a fetch callback to control SoC/FEM split for non-linearity, saturation, and quantization on nRF54L. Nordic provides no built-in model for simple GPIO / third-party FEMs.

Stine

RE: FEM issue

larry ding — Thu, 20 Aug 2026 06:41:38 GMT

Hi,

we changed ctx settle time and crx settle time to following values:

ctx-settle-time-us = <23>;
crx-settle-time-us = <5>;

but the test result shows no notable difference vs earlier data.

Any ideas? Thank you.

RE: FEM issue

stak — Tue, 28 Jul 2026 12:23:12 GMT

You are right. It seems the errata is not relevant there.

The RFX2401C uses a standard 2-pin control interface, which is supported by the nRF54L15 through the Simple GPIO FEM interface.
However, we only validate and test the Skyworks FEMs listed in our supported devices table. When using a different FEM, such as the RFX2401C, it is the customers responsibility to verify that the DeviceTree configuration and GPIO timing parameters meet the requirements of your hardware.

Based on standard configuration for supported skyworks ctx-settle-time-us and crx-settle-time-us should be assigned acoordingly to:
sdk-nrf/dts/bindings/radio_fem/skyworks,sky66112-11.yaml at main · nrfconnect/sdk-nrf
I see that you are setting it to 1us. Please check that these are configured properly.

Let me know how it goes!

Edit: Also check nRF54L section in https://nrfconnectdocs.nordicsemi.com/ncs/latest/nrf/app_dev/device_guides/fem/fem_simple_gpio.html

RE: FEM issue

larry ding — Tue, 28 Jul 2026 09:40:55 GMT

Sorry, but we don't understand why this errata may degrade range. It said the power emitted might exceed regulatory limits.

RE: FEM issue

stak — Mon, 27 Jul 2026 16:03:13 GMT

Hi,

I’m still looking into the issue, but one thing worth checking is whether your nRF54L15 is affected by this Errata: https://docs.nordicsemi.com/r/bundle/errata_nrf54l15_rev1/page/err/nrf54l15/rev1/latest/config_l15_56.html. (Check your revision) The errata specifically applies to the nRF54L15 and describes an issue with external FEM/PA operation, which can degrade range.

There may also be some impact from interference in the test environment, or other explanations, but I would recommend reviewing the errata and trying the proposed workaround as a starting point.

Stine

RE: FEM issue

skylu@telit — Mon, 27 Jul 2026 10:28:11 GMT

we use RFX2401C.

the FEM should have configured correctly. because with FWM device and SCAN and connection other BLE devices

here is the DTS

&radio {
fem = <&nrf_radio_fem>;
};

/ {
nrf_radio_fem: rfx2401c_fem {
compatible = "skyworks,rfx2401c", "radio-fem-two-ctrl-pins";

/* TXEN -> P1.10 ; RXEN -> P0.01 */
ctx-gpios = <&gpio1 10 GPIO_ACTIVE_HIGH>;
crx-gpios = <&gpio0 1 GPIO_ACTIVE_HIGH>;

ctx-settle-time-us = <1>;
crx-settle-time-us = <1>;

tx-gain-db = <25>;
rx-gain-db = <12>;
};
};

here is the config

# MPSL + FEM simple GPIO control

CONFIG_MPSL=y

CONFIG_MPSL_FEM=y

CONFIG_MPSL_FEM_SIMPLE_GPIO=y

RE: FEM issue

PaKa — Mon, 27 Jul 2026 09:57:26 GMT

Which FEM are you using and are you sure it is configured correctly?