How to measure the negative resistor of crystal while using nRF54L15 internal load capacitor?

[Webster Yang]

Hi all : 

I am trying to use the nRF54L15 built-in load capacitor for crystal but I do not know how to measure the crystal performance, such as driving level and negative resistor.

Could you help to explain how to measure them if I use the internal load capacitor?

[Kenneth]

Hi,

First make sure that the 32MHz crystal you have chosen fullfill the requirement stated in the datasheet:

https://docs.nordicsemi.com/bundle/ps_nrf54L15/page/_tmp/nrf54l15/autodita/OSCILLATORS/parameters.32_mhz_crystal_oscillator.html 

Make sure to follow our reference schematic and layout as closely as possible:
https://docs.nordicsemi.com/bundle/ps_nrf54L15/page/chapters/ref_circuitry.html 

In terms of how to find the optimal load capacitors, then I suggest that you build several radio tests firmware, but with different load capacitor values for each, e.g. different:

&hfxo {
load-capacitors = "internal";
load-capacitance-femtofarad = <15000>;
};

Using radio test firmware you can now execute start_tx_carrier command, by measuring the carrier frequency using a spectrum analyzer you can find which load capacitors that get you closes to the center frequency. Link to radio test:
https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/samples/peripheral/radio_test/README.html 

Kenneth