nRF9160 - antenna complexity and diversity

[borgestrand]

Hi, 

I'm comparing the nRF9160 DK and Thingy 91. The purpose is to copy the LTE RF design for a Band-20 application. The GPS will not be used.

Question: Is the complexity of the Thingy 91 dual RF muxes really needed? 

In our application, there will be two 800MHz antennas, one on-board and one connected by a 50R antenna cable. We will not use the DK's J1. Antenna selection will be in firmware. With the nRF9160 DK as a starting point (PCA10090 - nRF9160 revision 0.10.2), where would be the best place to insert an RF mux that connects either antenna to the nRF9160 and matching network?

Best regards, 

Børge

[Marjeris Romero]

Hi,

The LTE RF antenna pin on the nRF9160 is matched to 50 ohms. All the matching components you see on both the nRF9160 DK and the Thingy:91 are matching components only for the antenna, and are not part of the referense design for the radio. It's not possible to copy the LTE RF design of either of them unless you are using the same antenna parts, and also same dimentions for the ground plane and component placements around the antennas.

Usually antenna manufacturers release a reference design with a matching network for the antenna based on a given ground plane size (usually with the recommended minimum λ/4 in length) on their datasheets. When working with a different ground plane and components around an antenna you may need to alter this reference matching network to ensure optimal results. Usually you can contact the support team of the antenna manufacturers directly for help with matching if the design is .

Question: Is the complexity of the Thingy 91 dual RF muxes really needed? 

The Thingy:91 has a PCB size of 50mm x 50mm. The ground plane length is then around half of what is recommended for the lowest LTE bands. The RF muxes used for the Thingy:91 are part of the antenna matching network designed by the antenna manufacturer specially for the Thingy:91 and the considerations around the small ground plane. A reconfigurable matching scheme was needed in order to have operations on all LTE bands (698MHz up to 2200MHz) and GPS. Each band is matched by different L-type matching networks to give the best operation for each band. More information about this is given in this application note by Fractus: https://www.fractusantennas.com/files/AN_FR01-S4-210_Thingy91.pdf

If designing for only one band you will not need this approach.

In conclusion, the matching network for your design will depend on what kind of antenna you are using and the matching network recommended by their manufacturers. 

If using an external antenna (active and matched to 50 ohms) together with a PCB antenna, I will recommend going for an approach similar to the one used on the nRF9160 DK, with a coaxial connector with a switch, to switch between the cable antenna or the PCB antenna with its corresponding matching network.

If you want the antenna selection to be on firmware you can place a RF mux instead of J11, and use a normal connector for the cable antenna on one path, and the PCB antenna together with its matching on the other.

There are some examples of off-shelf antennas recommended for the nRF9160, together with some other recommendations for LTE antenna design in this whitepaper: https://infocenter.nordicsemi.com/pdf/nwp_033.pdf

Best regards,

Marjeris