Non-modulated emissions in 802.15.4

[Carton_]

Hi,

Two weeks ago, I've tested my new devices in an anechoic chamber. I'm using Fanstel BT840X modules. These modules are using nrf52840 for MCU/Radio and SKYWORKS SKY66112-11 for PA/LNA.

For the firmware, I'm using RIOT OS 2022.07 with a small modification to handle the FEM module of the BT840X. The radio used is 802.15.4.

During the tests, we noticed that the module transmits the desired frame but then emits a non-modulated signal in some conditions:

• We tried to change the TXPOWER. No change. Problem occurs at -8dBm, -4dBm and +8dBm (nrf52840 setting).
• We tried to slow the rate at which we emit the messages. In my software, if I emit a message each >=30ms, the issue disappears. If I emit a message more frequently than >=30ms, the issue starts to occur.

I'm pretty sure it's software related as we tested 2 different PCBs both using BT840X. One is using Zephyr OS as firmware for BT840X and the one causing these issues is the one using RIOT OS for its firmware.

This issue make us fail the power density measurements in EN300328. Luckily, as my device emits very rarely in real world use cases, I was allowed to lower the emissions rate during the tests to reflect a little bit more the reality of the devices and I could succeed the tests but I'm still curious why it happens. It would be nice to fix.

Below some screenshots of the spectrum analyzer we did to show you the issue:

1. TX - Temporal - Message each 30ms
2. TX - Temporal - Message each 5ms
3. TX - Spectral - Message each 30ms
4. TX - Spectral - Message each 5ms

Does anyone have a clue about why these non-modulated emissions are happening ? I'm not a radio expert at all so I hope everything is clear. Don't hesitate to ask if more information is needed.

Thanks!

[ketiljo]

It looks like it's the RIOS OS radio driver that does this. It seems to keep the TX on after the modulation is finished at the end of the packet. 

This is how a 15.4 carrier looks like when the radio test example is used to setup a carrier.50% duty cycle just to have some on/off toggling. 

The radio test example (https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/samples/peripheral/radio_test/README.html) uses the NCS radio driver.

[Carton_]

Hi,

Thanks a lot for your answer!

So could my issue be related to this part of the RADIO section in nrf52840 documentation:

As illustrated in Transmit sequence the RADIO will by default transmit '1's between READY and START, and between END and DISABLED. What is transmitted can be programmed through the DTX field in the MODECNF0 register.

?

RIOT OS driver doesn't use shortcut between END events and DISABLED.

[ketiljo]

Yes, this is most likely what happens. 

[Carton_]

Thanks!