Are the 333kHz spurs seen when measuring the transmit power in continuous wave mode normal?

Transmit output power can be measured using the JN-AN-1021 Module Lab Test Utility. The 'Transmit Power Test ' puts the device into a transmit state, generating a continuous wave signal. The JN5139 output spectrum contains spurs 333kHz either side of the fundamental. These 333kHz spurs are due to the synthesiser frequency used on chip. The spurs can be influenced by the loop filter components but should be ~30dBc down.

The Jennic JN5139 radio uses 333kHz (16MHz/48) as the reference frequency to the frequency synthesiser.  This enables an 'off frequency' VCO architecture to be employed, i.e. a VCO which is not running at the same frequency as the desired carrier frequency (2.4GHz). This avoids the effect of 'VCO pull' which occurs when the output signal generated by the internal power amp couples back (by radiation and/or by conduction) into the VCO and causes a phase shift that pulls the output frequency off the desired value.