I am wanting to use the AD9850 for a particular application to generate sine wave signals in the 100Hz to 5kHz range. I've purchased a AD9850 module from ebay which just looks like the standard ones that are floating around at the moment. I noticed that I get quite significant harmonics even at these low frequencies. I'm not sure how to design a filter for this because ideally I would just filter off everything above 5kHz but I've seen that when I set the AD9850 to 1KHz I get harmonics at 2 and 3KHz which are noticeable. So it seems like I would need to have a variable filter that gets rid of harmonics above whatever frequency the AD9850 is set at. From what I've read about this particular DDS chip I would not have expected harmonics at these frequencies but probably would expect it as the frequency gets into the MHz range.
Can any one offer any advice on this problem or suggest any simple modifications that could be made to the AD9850 module board to reduce, or ideally, get rid of these harmonics?