RE: nRF52832-QFAA design fails EN 300 328 (Radio) test; too high spurious radiated emissions at 4.9 GHz during RX

Andreas — Mon, 09 Mar 2020 12:34:54 GMT

Hi,

Yes. This is the VCO in the radio, the only source for ~4.9GHz in the IC, and errata 138.
Normally grounding the P0.25 and P0.26 into the center pad yields a similar improvement, yes. If you can re-assign the SPI lines to other pins, this is generally the safest option.
The inductor itself being underneath the IC is not a problem, but it will degrade the quality of the grounding of the IC.
This issue has shown a dependency on the quality of grounding, poor grounding of the IC or RF front-end exacerbates the issue.
Difficult to say, but more vias improves the grounding, which is typically a good thing. You might not need a 3x3, but at least one or two in the P0.25 corner area if you can find room for them.

The 12pF caps is one thing, but in order for this to work they need to be placed on the same layer, as close to the pins as possible. Furthermore as mentioned there must be an 'easy' way back into the center pad for the 4.9GHz signal that is filtered, meaning you likely need a solid ground plane without any signal or supply traces causing major detours. It is hard for me to judge this without seeing your design though.

Best regards,

Andreas