What is the purpose of the capacitor used in series with the TCXO output pin?

[AZube.2]

Hello guys,

I was inspecting the design files of the Nucleo board and could not understand the use of the capacitor used for the TCXO. In the image attached (nucelo design schematic file), a note says that the C31 is used in order to avoid stubs on XTA path when the same capacitor is not placed. I don't understand what it means, could someone help please?

[Nikita91]

The note does not say that C31 is for stub avoidance. This is a recommendation for the PCB designer who says do not stub. In other words C31 should not be on a branch of the signal, but directly on the line between the TCXO and the MCU.

What nucleo is it?

[Mike_ST]

>> What nucleo is it?

I guess Nucleo STM32WL55 (MB1389)

[AZube.2]

NUCLEO-WL55JC (MB1389D)

[AZube.2]

Yes, exactly.

[Tesla DeLorean]

Perhaps look at the loading expectations on the TCXO.

Then DC bias, wave form and Vp-p on the STM32WL side.

" A DC-cut capacitor is not embedded in this crystal oscillator"

[LCE]

10pF is surprisingly small.

So that STM32's input impedance on the oscillator input seems to be not very small, too...

[Uwe Bonnes · Accepted solution]

Have a look at UM2592 6.5.1 HSE clock (high-speed external clock). Stub avoidance is not mentioned in the text and the text sounds more reasonable

The TCXO feeds into XTALIN. XTALIN is an analog input with its own biasing and a high sensivity. A low power/level signal is needed to not overdrive and the capacitance to not disturb biasing