nRF52820 PCB RF Design

[backstreet.devisor]

Hi.

I am designing a 4 layer PCB (0.8mm thick) for an nRF52820 microcontroller. 

My board's stack:
L1 - Signal
L2 - GND
L3 - Power
L4 - Signal

I have questions regarding the design of the ground plane under the radio transmission line.

According to the datasheet recommendations, layer 2 and layer 3 should not have copper under the RF line.

But while calculating the RF width on the JLCPCB website, you have to select the ground reference layer (in my case it is layer 2) and the result of the scoring depends a lot on that.

Do I need to fill the space under the radio transmission line on layer 2 with copper (GND polygon)? Or should I continue to follow the recommendations from the datasheet?

I hope for your help.
Thanks.

[JONATHAN LL]

Hi,

Is L1 where you have the RF path\transmission line ?

If so then since you only have L2 as the GND plane then you should use that as the reference for the transmission line. So no cut out if you use L2 as reference GND. Fille the area with GND copper.  

Reason we do have a cut out in our 4 layer designs is that we use the bottom layer as GND also and have that as the reference for the transmission line, this helps reduce any stray capacitance in the design. 


Regards,
Jonathan

[backstreet.devisor]

Thank you very much Jonathan for your reply. It makes sense to me now.

I have another question.
On the L3 layer I have 3.3V power supply tracks, but I fill the rest of the space with GND polygon. Can I also fill the space under the RF line with GND polygon on layer L3?

Thanks.

[JONATHAN LL]

backstreet.devisor said:

On the L3 layer I have 3.3V power supply tracks, but I fill the rest of the space with GND polygon. Can I also fill the space under the RF line with GND polygon on layer L3?

Yes this can be done, but then you will have to adjust for this in the calculation of the RF path, but it is ok to do so. 

Regards,
Jonathan

[backstreet.devisor]

Hi. Thanks again.

What other changes need to be made to the calculations?

On the jlcpcb website there are no more additional parameters related to the ground reference layer.

[JONATHAN LL]

No other changes need to be made, its just that if you use L3 as the reference ground layer for the RF path then the thickness of the RF track will change slightly.  So as long as you have accounted for that there is no additional steps. 

PCB Antenna - How To Design, Measure And Tune 

https://devzone.nordicsemi.com/guides/hardware-design-test-and-measuring/b/nrf5x/posts/general-pcb-design-guidelines-for-nrf52-series 



Regards,
Jonathan

[backstreet.devisor]

I got it. Thank you so much for your help.