nRF9160 - GPS and Cellular shared antenna

[Vincent44]

Hello,

I am currently designing a custom board for my nRF9160 project, and I have some questions regarding the GPS / Cellular antennas.

For cost, space, and because the GPS will only be used once a day (max) for single fix, I would like to use a single antenna for both GPS and cellular.


On the the nWP-033 document (), section 4.3 provides some details on this subject.
However, there are not many details. My guess is that ANT pin is internally to the AUX pin ?


Looking at the schematic of the Thingy:91, that uses a single antenna for GPS + Cellular, the implementation is different.
On this board, an "RF multiplexer" is used to route the antenna to either GPS or LTE pins.
Also, the modem can drive the mux automatically, using the MAGPIOx, to select the optimal matching circuit, depending on the LTE band used.


This brings several questions :

- Why does the Thingy:91 implements several matching circuits, but not the nRF9160DK ? Should the nRF9160DK be considered less efficient in terms of RF ?
- On the Thingy:91, the RF mux QM12038 is now obsolete. Are the most recent Thingy:91 board still produced using old stock of this component ? Or do you have any equivalent to suggest ? Would the MAGPIOx pins still be compatible ?
- Why does the Thingy:91 does not implement the solution presented in nWP-033 document ?

I also found some old threads on DevZone concerning the nRF9160DK, but I beleive the hardware has been changed, so these are not longer relevant. Is this correct ?


Thanks a lot.

[Bendik Heiskel]

Hi,

- Why does the Thingy:91 implements several matching circuits, but not the nRF9160DK ? Should the nRF9160DK be considered less efficient in terms of RF ?

The size of the PCB, and in turn the size of the ground plane, in the Thingy:91 is very small. The multiple tuning circuits are used to combat the poor antenna performance at lower frequency bands caused by the small ground plane. The nRF9160DK have a larger ground plane which in turn improves the antenna performance on the lower frequency bands allowing for a much simpler tuning/matching circuit.

On the Thingy:91, the RF mux QM12038 is now obsolete. Are the most recent Thingy:91 board still produced using old stock of this component ? Or do you have any equivalent to suggest ? Would the MAGPIOx pins still be compatible ?

The Thingy:91 schematic have not been updated for a new RF switch. I don't currently have a suggestion for a equivalent switch, but a switch with the same number of outputs, the same control scheme and similar operating frequency should work. The MAGPIO pins should be compatible if the switch uses 4 GPIO inputs in the same way as the Qorvo switch.

Why does the Thingy:91 does not implement the solution presented in nWP-033 document ?

I don't know the exact reasoning for not using AUX pin, but I would assume since a external RF switch was already needed it was simpler to just use it for the GPS as well.

I also found some old threads on DevZone concerning the nRF9160DK, but I beleive the hardware has been changed, so these are not longer relevant. Is this correct ?

There have been changes to the GPS antenna and LNA used on the nRF9160DK. These changes was made in v0.15.0 of the DK so any threads related to version newer than this is still relevant.

 

 

I general we don't recommend using the antenna tuner design used in the Thingy:91 for custom boards as this is highly optimized for the antenna and PCB size and layout of the Thingy:91.

We can do a review of your board and give feedback on the design. To do this you have to create a private ticket and share the schematic and layout files, ideally in Altium format but gerber production files are also OK.

 

Best regards,

Bendik

[Vincent44]

Thanks for the details. This is great help.

Thanks for the info concerning schematic review. It might come handy once the design is more advanced.

For this project, I will be using an external antenna. The reference has not been selected yet, but I should be in the nRF9160DK case (good reception), so I shouldn't need a very fine tuning for each band.
After a quick look, it looks like there are not that many references optimized for GPS + LTE-M. My antenna will most likelly be good for cellular, but no great for GPS. This should be taken into account when selecting the filter/LNA. Correct ?

Overall, simplifying the Thingy:91 design, ends up with the attached imaged. Using a 2:1 RF switch, such as the SKY13453-385LF.

However, overall, it's just a more complex implementation of the recommended design from nWP-033. So it's probably easier and more reliable to use the nWP033 example ?

[Bendik Heiskel]

Sorry for the delay in getting back to you.

Vincent44 said:

After a quick look, it looks like there are not that many references optimized for GPS + LTE-M. My antenna will most likelly be good for cellular, but no great for GPS. This should be taken into account when selecting the filter/LNA. Correct ?

There are multiple external antennas that are optimized for GPS + LTE, the L1 frequency used by GPS is close to some of the LTE bands so a antenna optimized for both shouldn't be a problem. Any LNA designed for GPS/GNSS should be OK to use, most of them have a built in band pass filter so there shouldn't be any need to add a additional one.

Vincent44 said:

However, overall, it's just a more complex implementation of the recommended design from nWP-033. So it's probably easier and more reliable to use the nWP033 example ?

Using the internal loop back though AUX will simplify the design and reduce the BOM when GPS and LTE is sharing the antenna, so its the easier and possibly better solution in this case.

 

Best regards,

Bendik