For higher frequency I suggest you use a proper DAC rather than your home-made one. If you only want sine wave output then try this http://www.ebay.co.uk/itm/New-AD9850-DDS-Signal-Generator-Module-0-40MHz-Test-Equipment-/180820399452?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item2a19bc555c.
Yes,actually the concept of my schematic is very poor. (it sucks 
)
:
I will try this Digital Device System
Thank you very much for the suggest, very appreciated.
Best regards.