nRF52840 Antenna matching network

RE: nRF52840 Antenna matching network

JONATHAN LL — Wed, 08 Sep 2021 14:28:32 GMT

Hi,

In this case you can see a reply from Kaja.

For best tuning of the antenna i would contact the manufacturer to se what they recommend for your use case. We also do tuning of the radio and antenna here at Nordic, but if low pas filters are used, there is not much we can do for the radio.

in short:

The radio matching network and the antenna matching network

should be connected with a 50 Ohm coplanar wave guide:

The length, L, does not matter in theory, but a very long RF path is not optimal, but 15-20 mm is not a problem.

In your case the distance is short and you have a connector in the middel. The most importen ting is to reduce ground loops and have propper grounding. You dont have to measure this but you can use digital tools to calculate and incorporate it in your design. Altium has such tools.

As for chip antennas here is a short summery(link to more detail):

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.

As a rule we do not usually recommend specific chip antennas. The selection will depend very much on the end application design, the 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

Regards,
Jonathan

RE: nRF52840 Antenna matching network

Dominik Eugster — Wed, 08 Sep 2021 13:24:57 GMT

[quote userid="91187" url="~/f/nordic-q-a/79339/nrf52840-antenna-matching-network/328397#328397"]Have you made sure that the transmission line is close to 50 Ohm ?
[/quote]

Do I need to measure this without antenna only the transmission line? Then add the antenna and tune to 2.4GHz?

RE: nRF52840 Antenna matching network

JONATHAN LL — Wed, 08 Sep 2021 12:30:18 GMT

Sorry, we dont not have any application note or white paper chip antennas for any 52 series product.

We do have the Thingy91, it has a nrf52840 with a chip antenna that you could look at. Where a Johanson antenna is used. The nrf9160 DK also has similar design with QIAA package.

As for guides here is what we currently have:

(+) General PCB design guidelines for nRF52 series - Hardware design, test and measurement - nRF5 SDK guides - Nordic DevZone (nordicsemi.com)

https://infocenter.nordicsemi.com/topic/nan_040/APP/nan_040/nan40_intro.html?cp=16_1

Regards,
Jonathan

RE: nRF52840 Antenna matching network

Dominik Eugster — Wed, 08 Sep 2021 10:34:54 GMT

[quote userid="91187" url="~/f/nordic-q-a/79339/nrf52840-antenna-matching-network/328543#328543"] The Johanson low pass filter does work but i often find it does have a bit more loss then a vell tuned matching network. Bu make sure you follow the layout exactly to get the desired effect.[/quote]

Do you provide any application note or design guide for a chip antenna connected to the NRF52840-QIAA? The evaluation board has no chip antenna.

How much is the output capacity in ohms and j of the NRF52840-QIAA antenna port?

Regards,

Dominik

RE: nRF52840 Antenna matching network

JONATHAN LL — Wed, 08 Sep 2021 09:49:20 GMT

[quote user="Dominik Eugster"] I can't follow the layout, beacuse I have another schematic and application.[/quote]

To get the best possible RF results then i suggest that you try to follow the reference of the RF area as close as possible, even if you cant do it 1:1.
Preferably we do recommend that the RF path/transmission line is a grounded coplanar wave guide(GCPW). Adding proper via shielding.

[quote user="Dominik Eugster"]Anyway I changed the RF part with a low pass filter 2450FM07B0037. I think this will improve a lot the stability and dB, do you agree?[/quote]

The Johanson low pass filter does work but i often find it does have a bit more loss then a vell tuned matching network. Bu make sure you follow the layout exactly to get the desired effect.

The transmission line should also have a singel ground reference plane. So the layer beneath the signal path for the antenna should be a ground layer. To serve as the RF ground.

Regards,
Jonathan

RE: nRF52840 Antenna matching network

Dominik Eugster — Tue, 07 Sep 2021 16:00:45 GMT

Hi JONATHAN LL

Thanks for your answer. I can't follow the layout, beacuse I have another schematic and application.

The transmission line I can check and adjust to 50 Ohms, I will do this, then maybe it's better.

Anyway I changed the RF part with a low pass filter 2450FM07B0037. I think this will improve a lot the stability and dB, do you agree?

Regards, Dominik

RE: nRF52840 Antenna matching network

JONATHAN LL — Tue, 07 Sep 2021 12:45:32 GMT

Hi Dominik,

Best way to figure out the correct matching is to test, you can only get so far with calculating/simulating. When not following the reference layout the result will differ due to stray capacitance and inductance.

I do highly recommend that you follow the layout also.

Have you made sure that the transmission line is close to 50 Ohm ?

In the data sheet the chip antenna has a recomendation for layout as well.

When it comes to the radio matching components it is important to remember that the the components also serve to filter harmonics.

Regards,
Jonathan

RE: nRF52840 Antenna matching network

Dominik Eugster — Mon, 06 Sep 2021 19:01:52 GMT

this is the measurement without matching componets, only the antenna and the 0Ohm resistors until the ant pin of the nRF

a 0.5pF in series and 4.7nH shunt gives this diagram:

RE: nRF52840 Antenna matching network

Dominik Eugster — Mon, 06 Sep 2021 17:57:43 GMT

Here are the plots from the antenna without additional components