AD9857 System Clock fsysclk = 198MHz, configure the AD9857 output 21MHz, 42MHz, 60MHz,the output is very good, using a spectrum analyzer observed spectrum is also very good, but when the frequency is 65MHz , the sine wave distortion , There is a spectrum of 100MHz ,
The magnitude of 100MHz singal is similar of 65MHz .
then I observe SDO ,the data is written to the AD9857 are correct, and then modify the AD9857 output amplitude register, modify the AD9857 others' registers, but the 100MHz spectrum have always been! I do not know where is the problem!
I also found a problem that when the output frequency is less than 1kHz ,the out has nothing,
Looking forward to your replies!