AD9172

I am debugging the AD9172. I use two DDS to generate two ch IQ signals directly, and concat the signals to drive the DAC.  The tx_core_clk is 245MHz, the equivalent data rate is 245*4=980MHz.  The Channel NCO is bypassed and the Main NCO is set to 500MHz.

Observe the signal on the Spectrum analyzer， when the DDS frequency set to 34MHz(output signal is 534MHz),  I can see two spurious at 534+12MHz and 534-12MHz. When I change the frequency ,  I can see signal and spurious below:

Frequency      spurious

533 MHz       ±34 MHz    

534 MHz       ±12 MHz

534.5 MHz      ±1 MHz

535 MHz       ±10 MHz

536 MHz       ±32 MHz

537 MHz       ±54MHz

I want to know where these spurious come from and how to remove them.