Hardware design of the LTE/GPS RF part on custom nRF9160 based board

[bojan]

Hello, guys!

We started designing our custom nRF9160 SiP based board. We currently feel that the most challenging would be to design the part around LTE/GPS RF networks. In that sense, I made a comparison between nRF9160-DK Hardware files (version 0.10.2) and Thingy:91 Hardware files (version 1.4.0).

nRF9160-DK design approach:

RF part on nRF9160-DK board seems to be simpler and cheaper to implement. COEX0 pin of nRF9160 is used to disable GPS part when LTE modem is active. MAGPIOs are not used.  Two separate ports of the P822601 antenna are used for LTE and GPS. Finally, there is only one matching network for LTE RF signal.

Thingy:91 approach:

On the other hand, Thingy:91 is using QM1238 SP8T switches and MAGPIO0-2 pins of nRF9160 SiP to fine-tune the frequency band. Only one port of NN03-310 antenna is used for both LTE and GPS. COEX0 GPIO of nRF9160 is used to let nRF52840 SoC know that the LTE modem is active. This approach seems to be more expensive and more complex to implement. There is a great webinar that explains the design process though (link).

I would like to ask the following questions:

1. Which Frequency band is covered with the LTE matching network on nRF9160-DK board?
2. What are the benefits of using Thingy:91 approach, do we get global coverage with a single design together with the increased efficiency?
3. In Thingy:91 approach, if we select only bands B8 and B20, for example, will MAGPIOs automatically adapt to the desired bands selecting only appropriate matching networks on the board or they are gonna continuously run 000, 001, 010, 011, 100, 101, 110, 111 patterns?

Thanks in advance for your time and efforts.

Sincerely,

Bojan.

[ketiljo]

The main reason for using the two RF-switches on the Thingy91 is that the ground plane on the PCB is on the small side. This makes the antenna more narrow banded so the switches are used to tweak the antenna when the band is changed. On the DK, the ground plane is much larger so the antenna is able to cover the whole frequency range. 

1. The full frequency range of the nRF9160 is covered by the antenna. 

2. Yes, you can cover all bands with a small design. 

3.  The MAGPIOs will change depending on the frequency the modem is set to. If it's connecting on one band, the MAGPIO will set the correct tuning to cover that frequency. For instance, for band 8, at ~900 MHz, the MAGPIO will be set to 100. https://infocenter.nordicsemi.com/topic/ug_thingy91/UG/thingy91/hw_description/hw_antenna_tuning.html. For B20, it will use 101.

[bojan]

Thanks for your explanations, ketiljo.

We plan to have the board that will be even smaller compared to Thingy:91. Does that imply it is preferable we take the approach with RF switches in our case?

Cheers,

Bojan.

[ketiljo]

Yes, that will be recommended. Thingy91 is designed for world wide use and covers most of the LTE bands. If you can get away with a single band or two, you can greatly simplify the design. 

[bojan]

If we decide to cover a single band or two, we could get rid of RF-switches and cover the band(s) with the single matching network. Correct?

Is it possible for you (Nordic) to review our final schematic and layout designs and give us your feedback?

Thanks in advance!

[ketiljo]

With less bandwidth to cover, you can probably remove the switched all together. 

No problem doing a review of your design, just open a private case and upload your design there. Refer to this case.