Unnatural AD9850 Output Attenuation
I use the AD9850 board from NooElec, purchased on Amazon.  However, the unit I bought was described for up to 40 MHz. I used it to build Antenna Analyzer  



I measure voltage at the junction of C1 and R2. There is a Low Pass filter on the 9850 board.  So I am measuring after the filter. On the 9850 board, there is a 200 ohm resistor to ground before the LP Filter, and also another 200 ohm resistor to ground at output of filter. This contains schematic of what I believe is on my 9850 board. 

I measure + 0.300 volts peak, and - 0.300 volts peak (.6v peak to peak) very flat from 1 MHz to 30 MHz.  Then voltage drop off begins at 30 MHz up to 40 MHz.  I don't see the wobbling output similar to yours from 2 to 20 MHz.  

Not sure if this qualifies as an answer, but too large for a comment.  But at the least it might give you a comparison to your results.  

EDIT 1  :  The output of the AD9850 IC (not the board) is a current, so output voltage will be dependent upon the resistor you choose to use as a load on the AD9850 output.  "Pin 21 IOUT Analog Current Output of the DAC"