Thank you,sitti, I am not using the AD9857 evaluation board,but the Schematics is similar.but I did not use 82.5MHz elliptic low pass filter.
I used T1's output directly.I set W5 to + ,and the board have an output lower than 80kHz, but when the frequency is 1kHz,the wave distortion,and the amplitude is very lower(50mV,normally is approximately 1.0V).the 1kHz wave is as follows
The 79MHz signal's spectrum is as follows(Output Scale Factor is ”80“),Marker2 is 79MHz,Marker1 is 120.8MHz,When I adjust the system frequency(fsysclk), Marker1 have to follow the changes.Increasing fsysclk, Marker1‘s frequency is also increasing.
Seen on the oscilloscope 79MHz signal as shown below
I also found another problem ,when output scale factor is bigger ,the spectrum is worst.whten output scale factor is ”FF“,the spectrum is as follows;
If I use 82.5MHz elliptic low pass filter,79MHz signal will be better?
When frequency is 21MHz,the spectrum is as follows:
