50 ohm impedance for bluetooth antenna track on nRF52840

[Natalia_NPL]

Hello everyone!

I'm working with the nRF52840 microcontroller and I wanted to get a bluetooth antenna and since I know that the whole RF thing is very complicated, I was thinking about trying several antennas to see which one works best in my design.

My questions are the following:
+ To maintain an impedance of 50 ohms, how can I make the correct calculation of the track size? I have previously adapted the following guide:
devzone.nordicsemi.com/.../general-pcb-design-guidelines-for-nrf52-series

+ I have assumed the following, if I route the antenna on the top layer, the next layer should be GND for a good return, so should I place ground vias around the RF path?

+ I'm testing the following antennas: one on a PCB (with the same length as the evaluation kit), another on a chip (2450AT18D0100E) and I've also put the MM8130-2600RA2 connector to test another one just in case, is what I'm doing correct?

Finally, to select these antennas I use 0R resistors, but I understand that not all of them work the same at 2.4GHz, could you recommend one?

Thank you very much in advance

[mathiaso]

Hi, Natalia.

To maintain an impedance of 50 ohms, how can I make the correct calculation of the track size? I have previously adapted the following guide:
devzone.nordicsemi.com/.../quote]

To find the track dimensions of your 50-ohm coplanar waveguide you can use a calculator tool or if using Altium, you can configure the layer stack with an impedance profile in the Layer Stack Manager. There are lots of resources on this online, e.g., in the Altium documentation. However, it is always recommended to get in touch with your PCB manufacturer as well to make sure you get the numbers right.

I have assumed the following, if I route the antenna on the top layer, the next layer should be GND for a good return, so should I place ground vias around the RF path?

Yes, that's correct. The neighbouring layer should always be a ground layer. You can see how this is done in the reference layout and in the layout of the nRF52840, which is downloadable as part of the hardware files found here.

I'm testing the following antennas: one on a PCB (with the same length as the evaluation kit), another on a chip (2450AT18D0100E) and I've also put the MM8130-2600RA2 connector to test another one just in case, is what I'm doing correct?

It's often a good idea to add some extra length to the PCB antenna, to have something to work with during tuning. It is easier to cut off than to add length to the antenna. There is a white paper describing how to design the PCB antenna. You might also find this video helpful.

When choosing a chip antenna, one should choose one suited for the intended board shape and placement. As you're using a Johanson antenna, they have a Chip Antenna Selection Guide that may help you with this.

The connector can be placed in series with the rest of your RF path, as it is done on the DK, due to its internal switch.

Finally, to select these antennas I use 0R resistors, but I understand that not all of them work the same at 2.4GHz, could you recommend one?

I don't have a specific one to recommend to you. It might be that your PCB manufacturer can give you an option for what is both suitable and the cheapest.

Best regards,
Mathias

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