NRF52840 Poor Radio performance

[Gaétan]

Hi,

It's my first design with nRF52840 (I have make multiple design with nRF51 et nRF52 without problem).

I use SDK15.2 and BLE_peripheral_blinky example without modification.

See schematic:

C14, C13: 1pF
L1: 3.9nH
L2: 3.3nH
L3: 1.5nH

I have very poor radio performance:
 * With the ceramic antenna (JohanSon 2450AT42B100) the range is 30 cm !
 * With a wire connected in L2-L3 hot point (L2 L3 Not Monted) the range is 2 or 3 meters

I haven't implemented DC/DC converter, but I the blinky example, the DCDC isn't enabled...
There is a small problem on my 2 layers PCB: The maker have used bad thickness: 1.55mm FR4 and not a 0.8mm like the specification.

I very surprised by the poor radio performance... I think this is not only due to the bad thichness of the PCB board.

Regards,

Gaétan

[Andreas]

Hi,

 

The board thickness is not too important and not specified to be 0.8mm, if anything it is specified to be 1.6mm but as I said it is not that important.

Without having seen your internal/bottom layer, I am quite certain this has to do with your antenna. L2 =3.3nH and L3=1.5nH are valid for the Johanson test board, in a different board they will be different. See this whitepaper.

Obviously, if you e.g. do not have ground underneath, or if you have ground underneath the antenna, that will also contribute negatively. If you share your other layers also I might be able to say something.

C14 = C13 = 1.0pF and L1 = 3.9 nH are possibly not optimal for your design, but should not have an effect this large.

 

Best regards,

Andreas

[Gaétan]

Hi Andreas,

My board is composed by only 2 layers:

I need to precise, that I think there is a problem with generated power on nRF52840...

Because, when I try with the first Pi filter like nRF52840DK and a wire, the RSSI is very low (about -85dBm) compared with nRF52840DK (-40dBm) at the same distance.

Regards,

Gaétan

[Andreas]

Hi,

 

The bottom layer looks fine.

Soldering a wire on will likely not be the correct length and it will not be matched (missing a shunt cap). Regardless, you are welcome do sent me one of your boards to optimize the output power, match the antenna. We offer to do this for all our customers, just let me know if you are interested. If there actually is a problem with the nRF52840 I willalso work that out.

 

Best regards,

Andreas