Since I thought about making a DDS function gen out of a micro controller, I bought one one Aliexpress to see how a function gen like this is made and how it works.
I tell you right now. Nothing impressive came to light. The seven DDS functions (Noise, ECG, Revers saw tooth, saw tooth, Triangle, Square and Sine) have a maximum of 65khz output frequency each. But square, triangle and the saw tooth functions can only be use to around 15khz in my opinion. Depends on what you define as usable. When choosing the output frequency, you should select first the frequency step, which is a separate menu point.
The HF(high frequency output) double it's frequency beginning from 1Mhz with each step till it reaches 8Mhz. But already at 1Mhz the square wave is barely usable.
The interface is made of five tactile buttons. You have a real power switch too, not a soft power one. Potentiometers to select signal amplitude and offset are present.
The whole DDS function gen works from a nine volt battery. The voltages for the µC and the output OP-amp are generated from these nine volt one the board. That is done with a 7805 in TO92 and a ICL76605. The OP-amp is a NE5532 and the µC running the DDS function gen is a Mega16 with a R2R-network as DAC. Nothing fancy there.
Some datasheets are linked. Credits to the respective owners!
If you can't afford a good DDS function gen or a normal signal gen, buy this kit. As long as you just need a signal with a low frequency it'll work fine for you. But I if you want to do more and build some sophisticated projects don't touch this kit. It's basically just a toy.