Spurious frequency 12.8 Mhz. DCDC Converter ?

RE: Spurious frequency 12.8 Mhz. DCDC Converter ?

Sylvain — Thu, 24 May 2018 08:43:10 GMT

Hi Andreas,

I will open a private post.

Sylvain.

RE: Spurious frequency 12.8 Mhz. DCDC Converter ?

Andreas — Thu, 24 May 2018 08:15:07 GMT

Hi,

Something is definitely not right here, I would not mind having a look at your design. If you do not want to upload it here (will be visible to the public) you can register a 'private ticket' and just reference this case in it.

Still, in order to narrow down the source it would probably be beneficial if you could test the same application on a DK, or test your PCB with the radio test example in the SDK.

Best regards,

Andreas

RE: Spurious frequency 12.8 Mhz. DCDC Converter ?

AmbystomaLabs — Wed, 23 May 2018 22:29:23 GMT

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.

RE: Spurious frequency 12.8 Mhz. DCDC Converter ?

Sylvain — Wed, 23 May 2018 19:41:54 GMT

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

RE: Spurious frequency 12.8 Mhz. DCDC Converter ?

AmbystomaLabs — Wed, 23 May 2018 19:29:34 GMT

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?