Spurious frequency 12.8 Mhz. DCDC Converter ?

[Sylvain]

Hello,

When I analyse the output of my radio, I see that I have a spurious frequency at 12,8 Mhz.

I supply my custom board with a fidelity power supply so no problem with that.

I think it could come from the internal DCDC, what's its frequency switch ?

How can I disable it in the SDK15 ?

Maybe it is something else but no other idea at the moment.

Thanks for the support,
Sylvain.

[AmbystomaLabs]

If you are using the SD then through the API it is:

        err_code = sd_power_dcdc_mode_set(NRF_POWER_DCDC_DISABLE);  //
        APP_ERROR_CHECK(err_code);

If you are not running the SD then it is:

NRF_POWER->DCDCEN = 0;

Though the dcdc runs at 8MHz so I doubt it is directly caused by it.  Could be a mix of something.

Do you have an external amp or anything on it?  Is this a custom board?

[Sylvain]

Yes this is a custom board. But nothing else on the board than the nRF52840 and the required components...

[AmbystomaLabs]

It would also be good to see the signal with a more narrow RBW. Say 1kHz . If the skirts are wide (ie, lots of phase noise) then you know it is not from a clock or fixed duty device. The harmonics seem to indicate that this is true since they don't follow the fourier series for a square wave.

Also you should try warming it up and cooling it off and see if the frequency of the 12.98MHz changes significantly.  Anything off of the 32MHz should only move by about 10ppm over temp. This could just be an oscillation from a poorly matched output stage.