Friday, September 27, 2013

AD9850 DDS Module from eBay

Some time ago I purchased several modules from eBay AD9850. It's cheap and can be controlled almost with any micro by parallel or serial load mode. But when you see something cheap beware, there is some trick possible, especially when it come from China.

It took me several hours to read and understand how to calculate 40-bit word for loading desired frequency. I made small program that can be used for controlling HF receiver DDS VFO.

During experimenting with the module I have noticed that the board produce to much heat. The current was about 120mA that seems to be not OK. I rechecked all connections and resistors, but not found any reason. The module was stated for 5V power supply in the datasheet and also all circuits I found online use this module with 5V. My ham friend, who rapidly found the reason for exceed heating, said that probably 125MHz oscillator installed on this module originally are 3.3V and not 5.0V oscillator. We reduced the power to 3.3V and added LC-filter. The heat disappear and the current measured about 60mA that's seems to be a normal for AD9850 chip (20mA) with Oscillator (40mA).

  Unfortunately the module PCB designed in way that's it's impossible to separate AD9850 and Oscillator VDD lines. We need to run it on low 3.3V and lose some stability of chip start-up. Probably the blue potentiometer calibrating voltage on pin#15 may help with the issue.

I think this modules was originally designed for 5V Oscillators, but 3.3V was installed in the manufacturing. That's why it's was sold with cheap prices on eBay. Anyway we can use it. The stability of sine waves and module usability cost it's several bucks. The quality of total harmonic distortion are excellent for home-brew.