AD9951 - Inadequate Output


Attached is the schematic of my implementation of DDS - AD9951

I am using AD9951 - DDS and I have programmed it through SPI Interface successfully and I have been able to generate frequency of the range of 1MHz to 100MHz. I am observing the output using DSO - 100MHz Aplab with 1GSPS.

What I find is instead of getting 0.5Vp-p output from DDS, I am getting 0.25Vp-p output at lower frequencies say upto 10MHz. Beyond this this voltage keeps on reducing drastically and drops to around 20mVp-p. For no frequencies I get 0.5Vp-p.  I have removed R17 for this implementation and I am observing the output at the transformer output U4.6. All the readings are pertaining to U4.6.

I have configured ASF (Amplitude Scale Factor) = 3FFF. I am using RSET = 3.9K.

Please suggest me where are we going wrong.

Thank you all.

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