nRF51822 carrier-only test mode

[florentb]

I am testing the DTM mode on the nRF51822_QFAA, and I use the carrier-only test with the following command :

dtm_init();    
dtm_cmd(LE_TRANSMITTER_TEST, 39, CARRIER_TEST, DTM_PKT_VENDORSPECIFIC);
while(1);

The result, with a spectrum analyzer (radiated test), looks like this:

While this seems within spec, is it normal to have the two additionnal peaks, 165kHz apart from the carrier?

Or is it caused by a PCB / design issue?

I am sure these are not coming from my spectrum analyzer, as I reproduced it with several analyzers and also with several nRF devices with different antennas.

[bjorn-spockeli]

I have discussed this with one of our RF engineers: Its not the CPU activity itself, but the fact that the application resides in flash and is read by the CPU. The Flash and the Radio uses the same power supply, so if there is a lot flash activity this will cause noise which in turn gets modulated onto the carrier. This looks pretty bad when you set the chip to transmit with a constant carrier and measure it with a spectral analyzer with peak-hold over time, but will have very little impact during normal operation(i.e. with a modulated carrier), since these peaks will drown in the modulated carrier.

Thus, this is not an issue with certification, but I will convey the feedback to the SKD team.