Direct Digital Synthesis Device Controller

This project was submitted to Parallax by Mark Spencer, WA8SME

I recently came across an inexpensive kit that uses a Direct Digital Synthesis (DDS) chip to create a programmable and stable frequency source from DC to 30 mHz that has many uses around the shop and in projects. The signal precision, accuracy, stability, programmability, and RF output quality truly stimulates the imagination of the experimenter. The specifics of this DDS kit can be found at this web site: http://www.njqrp.org/dds/index.html.

The DDS chip is programmed by sending a 32-bit control word, and bam, there is the signal, clean and on frequency. The kit documentation points you to a site here you can download a Visual Basic program to program the DDS via the parallel port of a PC. I wanted to free the DDS from the PC to allow me to use the device as a more portable frequency generator and test instrument, and the BASIC Stamp microcontroller came to mind. One problem with programming the DDS is that it requires 32-bit non-integer math to calculate the control word, not an easy task for the BASIC Stamp microcontroller. Then along came the answer when Parallax offered the uM-FPU 32-bit Floating Point Coprocessor. This coprocessor is very powerful and takes care of many of the integer math limitations of the BASIC Stamp microcontroller; certainly it takes care of the algorithm needed to program the DDS. The result is a portable, battery operated, programmable signal generator.

My DDS controller uses the BASIC Stamp module and the DDS board as the center pieces. A keypad with a keypad encoder chip (MM74C922) provides user input. A 2X16 LCD display provides a visual display of the programmed frequency. And the uF-FPU does all of the heavy math work. The circuit diagram and pictures of the project are attached.

The program is straight-forward. The uM-FPU and the LCD are initialized on power-up and a greeting message displayed. The user keys in the desired frequency in hertz (yes hertz). Upon pressing the “#” key, or after entering 8 digits, the uM-FPU goes to work to calculate the required 32-bit control word to program the DDS. The BASIC Stamp microcontroller uses the SEROUT command the send the control word to the DDS and displays the frequency set on the LCD. The “C” key can be used to clear the entry if an error is made. The “*” key can be pressed to enter a new frequency.