I built a board with AD9958 - the way it is suggested in the spec sheet. I need to generate output signals in the range of 30 to 150MHz. I noticed as I change the frequency setting, the output amplitude varies almost two times.
I looked through the spec sheet, but I did not notice amplitude variation vs frequency spec. I would think that amplitude should not change with frequency; there are no specific filters on the DAC's output as far as I see..
I still suspect something wrong with my board, but in general - is there a notion for the output of DDS to stay at the same amplitude over the frequency range?
In a Partial DDS (No built-in DAC), the output is purely digital, in which the output level is constant across frequencies. However, in a complete DDS in which DAC is added (all ADI DDSs are complete DDS), the output amplitude is attenuated across frequencies. This is due to the inherent behavior of the DAC. In fact, it follows the spectral analysis as shown in the figure below:
A detailed explanation is provided by KennyG in this link: Why does the output signal level of a DDS appear to roll-off with frequency
If you want to have constant signal level at the output, you must employ a circuitry for it. A Circuit Note (CN-0050) shows a similar concept with this. You might want to look at it.
Thank you very much! It was very helpful!!