[nRF52840] Poor RF Performance on Custom PCB with Chip Antenna

[mkimhj]

Hi,

I have a custom PCB using the Johanson 2450AT18A100E chip antenna. For some reason, I'm getting extremely subpar RF performance and so I'm taking a second look at my RF path. Is my signal path out of the ANT pin of the nRF52840-CKAA through to the feed line of the chip antenna is correct? I am using the reference design for the nRF52840 CKAA and then feeding the output of the into the matching network from the Johanson datasheet.

Thanks

[ketiljo]

The schematic is correct. But the layout plays a major role as well, how does that look like? How have you gotten the value of the antenna tuning components, C24, L6 and L7?

[mkimhj]

The values for C24, L6, and L7 were taken from the Johanson datasheet:

www.johansontechnology.com/.../2450AT18A100.pdf

Below are screenshots from the layout:

[ketiljo]

Routing ground like this doesn't work well. The inductance in the tracks are too high. You need to connect the components C1 and C7 directly to the ground plane 

Remove TP19 as it will affect the antenna. 

The bottom side, under the RF section should be solid ground. No tracks or components here. 

You also need plenty of vias to connect the top and bottom ground planes together. 

When it comes to the antenna tuning components, they strongly depend on the layout, PCB size, proximity to metal and plastic etc. So mostly likely, the values from the antenna's datasheet doesn't apply to your board. Take a look at this whitepaper: https://infocenter.nordicsemi.com/pdf/nwp_017.pdf

[mkimhj]

Hey ketiljo,

Thanks for the tips. When you say C1 and C7, you're referring to C16 and L7 on my schematic, correct? 

TP19 was placed down so that the chip antenna could be reworked with a UFL connector for test and debug purposes. Plan on removing this for future iterations. 

The bottom side is actually a 1v8 power plane instead of a ground plane... Do you think this is what's causing the lackluster performance?

[ketiljo]

Yes, C16 and L7, sorry.

It's preferable to have a ground plane instead of a power plane. From a AC point of view, they are the same, but you can't connect the power plane to the ground plane the same way you could do with an inner ground plane. It's important to use plenty of vias to connect the different ground planes together for a low impedance ground connection.