S2-LP XTAL Design

[leomecma]

The ST Nucleo board X-NUCLEO-S2868A2 - Sub-1 GHz 868 MHz RF expansion board https://www.st.com/resource/en/data_brief/x-nucleo-s2868a2.pdf, use the XTAL configuration bellow

Someone know the reason of the inductor at input? Is this some kind of RF filter to 868MHz?

 

[FvM]

The same circuit is shown in an S2-LP application note, but without any explanation. https://www.st.com/resource/en/appl...r-the-s2lp-transceiver-stmicroelectronics.pdf

I can just guess that it's an absorption circuit to prevent radiated RF entering the crystal oscillator.

 

[leomecma]

The same circuit is shown in an S2-LP application note, but without any explanation. https://www.st.com/resource/en/appl...r-the-s2lp-transceiver-stmicroelectronics.pdf

I can just guess that it's an absorption circuit to prevent radiated RF entering the crystal oscillator.

I suppose too, I think the values must be at resonance to be a short circuit at oscillator input for RF frequency. XL value is much less than XC at 50 MHz crystal frequency to disturb its operation.

 

[vfone]

The rejected frequency of C13, L8, is 734.1MHz, which doesn't tell much about the reason of the circuit.
Maybe I missed something, but myself I don't see this notch filter in any of the circuits from ST datasheet or AN, other than this X-NUCLEO module.

 

[FvM]

The circuit is also shown on page 14 in the AN linked in post #2, inductor value is however unspecified "TBD".