nRF52840 continuous carrier TX test shows another peak +2MHz away from f_c, why?

[frank2608]

I'm running the nRF52840's radio (Raytec MDBT50Q-1MV2 module) in continuous carrier mode just to check some stuff with my RF spectrum analyzer. The analyzer shows a two peaks, one at f_center, and a smaller one +2MHz from f_center.

Where is this second peak coming from? And why is it upper sideband and nothing lower sideband?

Screenshot shows two differently positioned nRF52840 units performing the same test, but with different f_center, both of them have the same second peak at +2Mhz

The radio itself works fine, no bit errors when I actually send packets, RSSI look great. I am mainly just curious about this second peak, for educational purposes.

Thanks

[jackstones092023]

I think you should check the mode of operation of the radio when you start the continuous carrier mode. According to the radio_test.h file, there are four possible modes for the radio test:

• RADIO_MODE_CARRIER_TEST - Unmodulated carrier test signal

• RADIO_MODE_CARRIER_DUTY_CYCLE_TEST - Modulated carrier test signal with duty cycle

• RADIO_MODE_CARRIER_ED_TEST - Energy detection test

• RADIO_MODE_CARRIER_MODULATED_TEST - Modulated carrier test signal

Depending on the mode you choose, the radio will behave differently and may produce different signals on the spectrum analyzer. For example, if you choose the modulated carrier mode, you will see a modulated signal with a certain bandwidth and frequency deviation. If you choose the energy detection mode, you will see a signal that depends on the energy threshold and the carrier sense mode.

duotrigordle