BT840NE Amplification in ESB packet retransmissions

RE: BT840NE Amplification in ESB packet retransmissions

Håkon Alseth — Thu, 29 Jan 2026 12:26:39 GMT

Hi Yanis,

Glad to hear that this shows good results on your end as well.

Please note that this is a quick-fix from my side, and it is not fully verified. As mentioned, I have reported this internally to the ESB team.

Kind regards,

Håkon

RE: BT840NE Amplification in ESB packet retransmissions

Yanis — Wed, 28 Jan 2026 15:15:22 GMT

Hi,

Thanks for your quick response.

What you suggested worked, here is the new ppk2 current measurement, both the main transmission and retransmissions are at the same current levels :

Regards,

Yanis

RE: BT840NE Amplification in ESB packet retransmissions

Håkon Alseth — Thu, 22 Jan 2026 09:38:57 GMT

Hi,

Thank you for sharing this information. I have been able to reproduce the scenario, and it looks like the issue is related to how esb handles re-transmissions when FEM is enabled.

Looking at the PDN pin of the nRF21540, it indicates that the subsequent re-transmissions are not enabling the FEM:

I have reported this internally to the ESB team.

Calling "mpsl_fem_enable()" in esb.c::on_radio_disabled_tx_wait_for_ack() seems to fix the issue, but I cannot guarantee that this is does not have any unforeseen side-effects at this point.

diff --git a/subsys/esb/esb.c b/subsys/esb/esb.c
index 9a148554a3..004f794936 100644
--- a/subsys/esb/esb.c
+++ b/subsys/esb/esb.c
@@ -1473,6 +1473,7 @@ static void on_radio_disabled_tx_wait_for_ack(void)
                update_radio_tx_power();
 
                /* Transmission is armed on TIMER's CC1. */
+               mpsl_fem_enable();
                esb_fem_for_tx_retry();
                esb_ppi_for_retransmission_set();

Kind regards,

Håkon