Matching network know-how with nRF5340 + chip bluetooth Antenna

[frax84]

Hello,

i'm developing a custom board with nRF5340 (WLCSP package) that is very constrained in terms of space for a Bluetooth application. I looked on DevZone and i found a lot of interesting posts about design, suggestions about Matching network, antenna selection, etc.

Summarizing what i have understood, being space constrained, my best option is using a chip antenna. I used the selector guide from Johanson Technology as suggested in THIS post and approached to the "matching network" topic using the material of THIS post. Unfortunately, i have poor experience about RF circuits. For this reason, i would like to have a feedback from you to verify i didn't make any mistake:

This is the schematic i prepared for the Bluetooth antenna circuit:

Here, 3 parts can be identified:

1) Radio matching network: this has been designed using the Fig. 263 "Circuit configuration no. 2 schematic" for CLAA version of nRF5340 in the nRF5340 datasheet (v.1.5)

2) RF line: this has a directive to be a 50 Ohm line

3) Antenna matching network: the chip antenna i chose is a 2450AT18B0100001E from Johanson Tech for corner mounting. The design of this section is based on the reference circuitry of the datasheet, depicted here below:

This is what i did until now. I have several questions:

1) Is the overall design ok? Did i miss any concept or make any important mistake?

2) The radio matching network is based on Fig.263 of nRF5340 datasheet (v.1.5). I noticed that all the reference designs for QKAA package are missing L4 (ref my schematic), while all the CLAA package ref designs have it. I suppose this is due to the difference in the wire bonding the chip on the two different packages. Can you confirm this or explain me why it is like this?

3) If i understood correctly,from a PCB layout point of view, i need to place Radio matching network the closest possible to ANT pin of nRF5340 chip, the Antenna matching the closest possible to the Antenna chip and the two networks need to be connected through a 50 Ohm co-planar wave guide. Did i miss anything?

4) I need to place a battery to supply power to the board. Can the position of the battery affect heavily the reliability or the range of the bluetooth? The final board should be pen-shaped, like the picture below:

where the green part is the board with the nRF5340 chip (the black square), the red part is the chip antenna location and the blue one is the battery location. Because of the application, it is very hard to switch the position of the board and the battery.

Thank you,

frax

[mathiaso]

Hi, Francesco.

Julius' comments are mostly right. Thank you, Julius! I just assume he meant mm, rather than cm, regarding the PCB antenna length. As he mentions, we have a white paper on PCB antenna design and you might also find this video helpful.

1) Is the overall design ok? Did i miss any concept or make any important mistake?

The schematic looks good so far. When continuing with your layout, use the nRF5340 reference layout and the layout of the nRF5340 DK found in its hardware files.

2) The radio matching network is based on Fig.263 of nRF5340 datasheet (v.1.5). I noticed that all the reference designs for QKAA package are missing L4 (ref my schematic), while all the CLAA package ref designs have it. I suppose this is due to the difference in the wire bonding the chip on the two different packages. Can you confirm this or explain me why it is like this?

Yes, there are internal bonding differences between the two packages. The QKAA is larger in size than the CLAA and thus has longer internal bondings. Hence, it makes sense that the CLAA has more inductance added at its ANT pin than the QKAA. This has a good effect on filtering out harmonic frequencies.

3) If i understood correctly,from a PCB layout point of view, i need to place Radio matching network the closest possible to ANT pin of nRF5340 chip, the Antenna matching the closest possible to the Antenna chip and the two networks need to be connected through a 50 Ohm co-planar wave guide. Did i miss anything?

You're absolutely right. Follow the reference designs of both the SoC and the antenna you end up choosing, and you should be heading in the right direction. You'll probably also find this guide helpful.

4) I need to place a battery to supply power to the board. Can the position of the battery affect heavily the reliability or the range of the bluetooth? The final board should be pen-shaped, like the picture below:

where the green part is the board with the nRF5340 chip (the black square), the red part is the chip antenna location and the blue one is the battery location. Because of the application, it is very hard to switch the position of the board and the battery.

As Julius says, I'd also suggest rotating the green part to point the antenna away from the battery.

Best regards,
Mathias

[frax84]

First of all, thank you to both of you JT898 mathiaso ,

JT898 said:

I have a 10cm x 4.5cm PCB with a PCB single monopole antenna of only 20 cm (23cm is the suggested length for 2.45GHz PCB antenna from Nordic's NWP 008)

Even assuming you're speaking about "mm" and not "cm", this is still too much for my application. When i said "pen-shaped" i also meant "pen-sized".

JT898 said:

You will need to proof your 50 ohm impedance by probing the antenna and measuring the SWR (<2.0 is good) and see how close you are to the 50ohm impedance on 2.400, 2.450 and 2.480 GHz with the battery next to it of course. It's crucial you test this with an enclosure that's close enough to production

I'm very noob about RF know-how, so i have not a practical idea on how to perform the checks you suggest, but i'll study a bit about it, thank you!

mathiaso said:

The schematic looks good so far. When continuing with your layout, use the nRF5340 reference layout and the layout of the nRF5340 DK found in its hardware files.

Not an easy task for my skills, but this is how intend to proceed. When possible, i avoid to re-invent the wheel

mathiaso said:

You'll probably also find this guide helpful.

It is, but just so you know, there are a couple of dead links in the guide. It would be nice to fix them

JT898 said:

Can you rotate the green block so the red antenna faces to the left of your picture?

That's a good point. Need some verification check, but this should be feasible.

Thank you all for your unvaluable support.

Frax