nRF54L15 QFAA — C9 Ground Reference with Custom PCB

[Tom_H]

Hi,

we are designing a custom board based on the nRF54L15 QFAA (QFN48) using a chip antenna with coplanar waveguide feed on a 4-layer PCB.

In the Nordic reference design, C9 is routed to the bottom ground layer, which also serves as the antenna path reference. In our stack-up however, achieving 50 Ω on the coplanar feed line requires using Layer 2 as the RF ground reference instead of the bottom layer.

Question: Should C9 be connected to Layer 2 (our local RF ground reference) or to the bottom ground layer as in the reference design?

Thanks in advance!

Best regards
Thomas

[Szabolcs]

Hi Thomas,

Yes, the C9 GND should be connected to the bottom layer polygon with vias. This is not due to reference grounding, but to have a very small series inductance with the capacitor for harmonic suppression. (This also means that the current from C9 to the exposed pad flows through the bottom layer.)

[Tom_H]

Hi,

thank you for the quick response! This clears things up for us.

Best regards
Thomas