RP2040 connectivity board mounting

[AlanH]

Hi, I have mounted the connectivity board to a baseboard via the 26 way header to allow me to connect some sensors to the connectivity board. However I have noticed that when the connectivity board is mounted to the baseboard the SARA modem stops functioning reliably. Unplugging the connectivity board from the baseboard but leaving it sitting on top of the baseboard still causes the SARA modem to misbehave. I have come to the conclusion that there is some form of RF coupling happening between the connectivity board and my baseboard as rotating the connectivity board into a vertical plane removes the problem. The baseboard only has a few passive components installed but is fully covered in copper pour. Is there any guidance on how the connectivity board should be mounted to another baseboard. I.e. keepout areas under/ around the connectivity board etc.

[Pontus]

Hi Alan,

That is an odd behavior that we have not seen before. Let me check this and get back to you.
In the mean time can you make sure that you are not running ground plane under the WiFi antenna and the cut out in the PCB for the antenna.
Are you able to share a picture of your board ? That would help us to reproduce the error. And maybe share what is happening with the SARA modem.

Regards
/Pontus

[AlanH]

Hi Pontus,
I have attached a pic of the baseboard. I have excluded the connectivity board from the pic so you can see the ground plane on my baseboard. Reading your reply I can say I do have ground plane under the Connectivity module. I have tried inserting a ferrite sheet between the module and baseboard and this has made a small improvement so maybe the problem is with the ground plane. I haven't been able to find any documentation on ground plane keep out areas for the connectivity module.
Regards
AlanH

Screenshot 2024-12-12 112924.png (153.47 KiB) Viewed 90 times

[Pontus]

Hi,
Thank you for the picture that clears it up a little bit.
As a general rule there should not be any ground plane below the WiFi antenna area but I realize that this is not something we have documented. I will make sure to fix that during the day and reply here.

But having said that, I have tried to replicate what you are seeing by running the board sitting on top of a another board with a full ground plane (I also tested with connecting the ground between the boards) and have not been able to see any problems. Can you give me a more detailed description on what you are doing when the modem misbehaves and what it is that happens with the SARA modem. Are you running WiFi at the same time as you are running LTE data traffic. How are you powering the board ?

Regards
/Pontus

[Pontus]

As promised here's an updated dimensions drawing with the required copper cut out region for the WiFi antenna. This drawing is based on having the board mounted with 3mm stand offs from the base board. If the distance is increased from 3mm's the cut out region needs to be extended with the same amount as the height is increased.

dimensions.jpg (159.81 KiB) Viewed 22 times

Oh and make sure to use a plastic stand off and screw for the pillar just above the WiFi antenna. A metallic screw will interfere with the radiation from the WiFi antenna.

And exactly like you have done on your drawing the Connectivity board need to be mounted on the left side of the base board so that the WiFi antenna (and USB connector) lines up with the edge. Preferably it should have been mounted as close to the top edge as possible so that the copper cut out creates an open area in the top left corner but the way you have done it is OK.

/Pontus

[AlanH]

Hi Pontus, Firstly thanks for looking into this and thanks for the updated placement notes. To answer your questions we are only using the LTE connection at present. WiFi and Bluetooth are not being used. When we started our testing we powered the connectivity board via the USB connection and the board was not attached to our baseboard. We were able to get the connectivity board connected via LTE to a MQTT broker and maintain a solid connection . But when we mounted the board onto the baseboard we noticed that we struggled to get a LTE connection to the network provider and also if we did get a connection it was very flakey and would drop out after only a few seconds. At this point we were powering the board from a small 500mA DC/DC converter on the baseboard. I thought that maybe this wasn't providing enough power during Tx so replaced it with a 1500mA supply and then directly from a benchtop PSU. Neither of these changes improved our connection success rate. Powering the board via the USB connector also made no difference. Removing the connectivity board from the baseboard resolves the issue. Setting the connectivity board on top of the baseboard reinstates the issue. We have been able to improve our connection success by fitting a thin ferrite sheet between the connectivity board and baseboard and this does improve the connection success rate but its still not as good as just removing the baseboard completely. So I think it is definitely an RF issue. I will remake the baseboard using the suggested keep outs and see if that makes a difference.

Thanks again for helping
AlanH