Matching network and antenna selection

[borgestrand]

I'm designing the nRF52840 into a new product with BLE, LTE and GPS.

Could you please point me to a recommended matching network and antenna solution for the nRF52840? 

We will use a 2:1 MUX to select between a board mounted antenna and external antenna connector. So there must be AC coupled 50Ohm nodes at the mux input and outputs. 

The internal antenna could be PCB type or an SMD component. I don't have a preference. 

Should I use a SAW filter in this setup? 

I may have access to a VNA. What would be the best method to tune the antenna? With our without the radio mounted?

In the board stackup I can have the reference ground plane either 0.1mm or 1.5mm below the RF signal layer. This is a 4L stackup with microvias. The outer insulation layer thickness has a fairly large tolerance. I plan to use L3 as ground reference to allow for a few non-critical signals routed on L4. The question is whether I should use the much nearer L2 as ground reference as well. 

Best regards,

Børge

[Kaja]

Hi Børge,

Could you please point me to a recommended matching network and antenna solution for the nRF52840? 

Antenna choice is complex and depends on many factors including technical and commercial considerations. Here are some possible types you can consider: 

Monopole, printed PCB antenna: This is easy to make and easy to tune, you also only need one impedance matching component, so it’s cost effective. Here the spacing is the issues, you need to make it about 23 mm long needs a minimum of 5 mm clearance to the ground plane. High bandwidth, making it fairly resistant to detuning. Link to our whitepaper: https://infocenter.nordicsemi.com/pdf/nwp_008.pdf?cp=12_18

Meander antenna, printed PCB antenna, ex. our dongle antenna design: Requires a smaller area than the monopole antenna, but usually requires a pi-network for tuning in addition to length. Lower bandwidth than a quarter wave monopole antenna. Here is a link to our nRF52840 Dongle design files as an example of this: https://www.nordicsemi.com/Software-and-Tools/Development-Kits/nRF52840-Dongle/Download#infotabs

Chip antenna: Higher BOM, but the antenna is small. The downside is that it usually has less gain. It requires a matching network, based on the vendors recommendations. It has a lower bandwidth than a quarter wave monopole antenna so it can be sensitive to detuning.

Reducing antenna size most often results in reduced performance. Some of the parameters that suffer are:

• Reduced efficiency (or gain)
• Shorter range
• Smaller bandwidth
• Distorted radiation pattern
• More critical tuning
• Increased sensitivity to component and PCB spread
• Increased sensitivity to external factors (“body” effect, ground plane etc.)

It is often better not to reduce antenna size too much, if you can avoid it.

As a rule we do not usually recommend specific chip antennas. he antenna vendors can assist on choosing the right antenna for a specific design, for example Johanson has a useful tool that helps with this selection: https://www.johansontechnology.com/chip-antenna-selection

Matching network for the radio and monopole printed antenna:

Matching network for the radio and meander and chip antennas:
 (J1 is a RF test connector, not required) 

Should I use a SAW filter in this setup? 

 - Yes, see our nRF9160 DK as a refence of combining BLE, LTE and GPS on one pcb. 

I may have access to a VNA. What would be the best method to tune the antenna? With our without the radio mounted?

See our Antenna tuning white paper. You can tune with and without the radio mounted, depending on the space limitation. We are also happy to help you with antenna tuning. 

In the board stackup I can have the reference ground plane either 0.1mm or 1.5mm below the RF signal layer. This is a 4L stackup with microvias. The outer insulation layer thickness has a fairly large tolerance. I plan to use L3 as ground reference to allow for a few non-critical signals routed on L4. The question is whether I should use the much nearer L2 as ground reference as well. 

 From our PCB guidelines: 
"A PCB with a minimum of two layers, including a ground plane, is recommended for optimal RF performance. On PCBs with more than two layers, put a keep-out area on the inner layers directly below the antenna matching circuitry (components between device pin ANT and the antenna) to reduce the stray capacitances that influence RF performance."

More ground us good. 


Feel free to make this case private or make a new private ticket, upload your HW files to get a full HW review.

Best regards,
Kaja

[borgestrand]

Hi Kaja, 

Thanks for sending the schematics! I'll take it and add AC coupling at the MUX. 

I'll most likely end up with meander, your J1 to be on the safe side, grounding on L3, and SAW filter.

Thanks for your suggestion on turning it private. I'll do that before sending you the gerbers. 

Best,

Børge