I use the reference design of fmcomms1 with ml605, and send white gaussian noise signal data through DDR->DMA->DAC. When I observed the air signal with the spectrum analyzer，what I observed is below:
The parameters is : DAC sampling rate :100MHz, Tx frequecny :2.2GHz.
When I send DDR DDS data through DAC, which is sine wave data(20MHz), setting the DAC sampling rate = 591.52M(which is recommended) and Tx RF frequency = 3.0GHz, there is still have harmonic. The picture I took photo is below:
My question is: Why are there harmonics? Where does it come from? Shouldn't the harmonics be filtered and removed before sending in antenna? Because the harmonics will affect other bands.