the issue about AD9910,output signal amplitude is unstable

I'm using AD9910 to generate a sinusoidal signal

The output signal amplitude decline sharply when the output signal frequency over 200MHz

when the output signal frequency exceeds 320MHz,the amplitude decline to zero,the output signal is gone. when the frequency behind 200MHz,the amplitude of the output signal is 330-480mv.

1. I already enabled the inverse sinc filter,if I disabled inverse sinc filter ,the output signal amplitude is dropped from 0.7V to 0V. same with enabled inverse sinc filter.

2. VCO frequency range bit setting 101 already

3. I changed the output filter and the amplitude still dropped sharply

what should I do? thank you