Identifying fault in a custom NRF9161 board

[Jakub Nagy]

Hi everyone,

I have a custom board using the NRF9161. Its hardware is loosely based on the design of the Conexio Stratus Pro and I’m using the Stratus board definitions/configuration files.

So far, I’ve made sure that:

1. The MCU can communicate with the modem.
2. The modem is updated and is running the 2.0.1 modem firmware instead of the preloaded PTI.
3. I ordered a development IMEI number from the nRF Cloud and written it into the OTP memory of the modem.
4. The device can read the IMEI and SIM ICCID in the application.
5. The code and configurations are working on a development board.

My testing routine consists of running the AT client example and using the following commands:

1. AT%XSYSTEMMODE=1,1,0,0 - to enable both NB-IoT and LTE-M
2. AT+CFUN=1 - to turn on the modem
3. AT%XICCID - to validate SIM status
4. AT+COPS=? - to run network scan

The network scan always returns an ERROR.

I also bought the Conexio Stratus Pro development board to validate that the code is running correctly. Using the same code and antenna, the development board is able to connect to the network and my custom board isn’t.

I’m pretty sure it isn’t a hardware problem, as in the chip isn’t soldered correctly as I can measure the same resistance between ANT and GND on the development board and the custom board, meaning there shouldn’t be a short at least. I also reworked and replaced the chip several dozen times at this point to account for this. When analyzing the power supply, the voltage held at 3.27V with a ripple of 50mV, so it should be well in the operating range.

I’ve been debugging this for over 3 weeks now with no avail. Do you have any ideas? Am I missing something? Thanks.

[Charlie]

Hi Jakub,

Thanks for reaching out to us regarding this issue. After reading your description, I’d like to share some thoughts.

Since you have the capability to replace chips, have you tried swapping nRF9161 SIPs between your custom board and the Conexio Stratus Pro development board while using the same SIM card?

This kind of cross-testing should help identify the root cause of the issue. I suspect the problem might be related to your custom board, particularly its RF performance. When manufacturing the board with a specific PCB manufacturer, RF tuning may be required, but this is jus suspection.

Additionally, providing logs of your AT commands and a modem trace would be helpful for further analysis.

Best regards,
Charlie

[Jakub Nagy]

Hi Charlie,

thanks for the quick reply.

I did go and re-check the impedance matching on our PCB design. We use a single strip coplanar waveguide with no matching components, the same as the Conexio Stratus Pro board. It turns out that I did in fact use an incorrect trace width. The updated calculation is below along with the trace geometry:

As you can see the impedance should be 8 ohms off the target. Do you think this is significant enough to cause such severe signal loss on this short trace? Waiting for a new board to get manufactured and assembled is a stretch right now.

I might try to measure the impedance with a NanoVNA but with no testpoints it might be difficult. I can also try to cut the trace and connect a coax to the start. Or I can try to put a series cap/inductor to try to compensate. Do you think any of these might work?

Lastly, I can try what you suggested, swapping the SoC between my board and the development board but it might be risky, cause I worry the development board won't work after either.

[Charlie]

Hi Jakub,

Good to hear that you found improvement in the hardware.

I'm not a hardware expert, but I know that some PCB manufacturers offer impedance control. For example, Controlled Impedance PCB Layer Stackup- JLCPCB. You may want to check with your manufacturer to see if they can provide guidance or assistance with this.

I understand your concern. You can start by collecting logs and modem traces from your custom board. The signal strength might give some clues about your antenna performance. You can check this using AT+CESQ. See: Nordic DevZone Discussion for the expected RSRP.

Best regards,
Charlie

[rajeev]

Jakub Nagy  this is Rajeev, creator of the Conexio Stratus series boards, and I'm happy to hear that our board turned out to be helpful and that you finally managed to fix the antenna path and size issue.