Last edited: 10.1.2022
I have build a DDS generator with a Arduino Pro Mini and a cheap AD9850 module.
Both can easily be found on ebay.
It is based on a description by AD7C.

By the push button you select the tuning steps:
10 Hz, 50Hz, 100 Hz, 500 Hz, 1 kHz, 2.5 kHz, 5 kHz, 10 kHz, 100 kHz, 1 MHz.
With the encoder you adjust the frequency between 1-30 MHz.

The output is terrible! Very much distortion and harmonics.
Others say this circuit works great, maybe I did something wrong.
So I never got this finished by building it into an enclosure.

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Update 18.03.2018:
I made some changes to the SW so the screen looks like this:

I changed the step rates so only these are available: 10 Hz, 100 Hz, 1 KHz, 10 KHz, 100 KHz and 1 MHz.
I added a push button so the step rate can be changed up and down,
I added a arrow up in line 2 to show which figure is changed by the encoder.

Update 22.02.2020:
Build it on a peace of wood so it is usable on the bench.

Here you find my schematic, (Eagle), still only 5V supply and no 12V regulator.

The output still is horrible and a lot of noise is heard beside the selected frequency.

You can download my arduino sketch: dds_ad9850. (ino file in zip format).

Here is a much better solution than mine 🙂