Drive strength for XTAL-P/N on ESP32-C6 (or in general)

[adibub]

Hi all,

Designing a ESP32-C6 board and the HW refrence manual states under section 2.4.1 that

In order to reduce the impact of high-frequency crystal harmonics on RF performance, please add a series component (resistor or inductor) on the XTAL_P clock trace. Initially it is suggested to use an inductor of 24 nH, and the value should be adjusted after an overall test. Note that the accuracy of the selected crystal should be within ±10 ppm.

If the drive level of the MCU is within the spec of the crystal manufacturer (100 uW) Rs would not be needed, but i dont see the drive level mentioned anywhere in the datasheet.

If i calculate for Rs using Rs = 1/2pi*f*C2 @40 MHz and 26 pF i get 153 ohms.

To my knowledge I have never seen a inductor being used in series with the C1.

[ESP_Sprite]

Note that the -C6 is not only a MCU, which indeed wouldn't need such specifics on the crystal. The thing is that the C6 also has a radio, which is an analog part and which is influenced in an analog way by the quality of things like the clock signal. Not having that inductor there will likely work if all components are within spec, but you can optimize the range and side emissions by making everything work juuuust right.

[adibub]

Thanks for your reply, i understand why one would need a stable clock for emission and RF range purposes.

Is there any good way to approx. the damping resisitor?

I can just add the 24 nH inductor and optimize later on when i have the board if needed.

[ESP_Sprite]

If I read the guidelines properly, it's either a resistor or an inductor, so I'd simply pick a footprint that can accommodate either and initially indeed stick an inductor there. The proper way to adjust it later, as far as I understand it (I'm not an RF magician) is to use the RF testing firmware and a VNA to measure the 40MHz and tweak the component for frequency andlowest harmonics.