I see high radiated spur within the frequency range of the Si4x6x RF chip VCO signal. How can I suppress this?
In RX only boards due to the lack of additional filtering the VCO signal might be radiated via the antenna too. In that case some filtering between the antenna and the matching balun network is recommended.
Moreover, the filtering of the traces connected to the SDN, GPIO 2, 3 pins (and the DC supply line of a TCXO) is also suggested at the VCO frequency (not only in case of RX only boards).
Use cases where applying additional filtering on SDN, GPIO 2, 3 (and the DC supply line of a TCXO) is strongly recommended:
The actual level of this radiated spur can vary with the actual board design.
Note: Not all Si4x6x reference designs use this additional filtering on SDN, GPIO 2, 3 (and TCXO VDD supply) lines. If possible, it is recommended to apply the additional filtering inductors on all customer designs. If BOM cost is critical, these elements can be eliminated from the design, but be aware that depending on the actual PCB layout structure, increased radiated spur in the frequency range of the VCO signal might appear.
The SDN and GPIO 2, 3 filtering on Silicon Labs reference designs is shown in the following figure:
The inductor value should be selected in a way to ensure enhanced suppression at the VCO frequency, so inductors that have SRF at the ~ 3.5 GHz frequency range.