Project finish

The keypad used for waveform mode change and frequency value input, and the two knobs used for amplitude and DC offset control. A SD/MMC card slot is next to the knob.


The DDS with two output connectors, one provides normal signal, another output filtered signal.

Demonstration

Arbitrary waveform output, 1kHz.


Sawtooth waveform output, 1kHz.


Square waveform output, 1kHz.


Sine waveform, normal and filtered outputs, 1kHz.

Final testing & assembly

Final testing before assembly.


Making connection to each boards, such as DDS main unit, power supply, LCD display and keypad. The IDC ribbon cable used to connect the LCD display.

Case design

To design a plastic case, the place arrangement of each boards, keypad and LCD display are very important. For some critical dimension, a tool like Vernier Scale is useful for measurement.


The acrylic boards layout was designed with the AutoCAD software. Cutting and drilling those acrylic boards performed by the laser engraving machine.


Using the Chloroform (CHCl3) to glue the acrylic boards.

SD/MMC card socket

The important thing before soldering the SD/MMC socket, it should be drilling two holes to the PCB, they can fixing the socket itself and against insertion force from the card.

Soldering components

After the PCB developing and etching process, the next step is soldering components.


Using a magnifier to help for soldering some small components, such as SSOP and TQFP packages.

3D PCB design

"EagleCAD" + "Eagle3D" + "POV-Ray" = Beautiful 3D rendered images. It is very useful before the actual PCB fabrication, I used this "virtual layout" to check the component orientation and the board space is enough or not.


Another small board is a power supply module.

PCB design

EagleCAD 5.9.0 used for the schematic drawing and PCB layout.


This screen capture shown many "air wires", they still need to route perfectly on PCB.

Arbitrary waveform - successfully developed

Arbitrary waveform

There are many problems when implement arbitrary waveform; first, the flash memory (ROM) inside of the AVR microcontroller is used to store the program and constant waveform patterns, it cannot be change; second, the internal data memory (RAM) is used dynamic address allocation method to store data, also, it's not enough space. (Screen capture above shown when storing sine wave pattern to the data memory, it can't reconstruct completely.)


One of the solution to fix this problem is used the external memory - SD/MMC card. It provides a large capacity to store the different waveform pattern files, but it can't run the waveform directly, so I need design a program to load the waveform file first, from SD/MMC card to the EEPROM memory that inside of AVR microcontroller.

Software simulation

For the DDS project software testing and debug, a software simulation suite was used, it can provided a better way to troubleshoot and find out any error in the program.


The VSM (Virtual System Modeling) combines mixed mode SPICE circuit simulation, animated components and microprocessor models to facilitate co-simulation of microcontroller based designs. The most important feature its ability to simulate the interaction between software running on a microcontroller and any analog or digital electronics connected to it.

Waveform smoothness

After analogue signal generated from the DAC (Digital-to-Analogue Converter), it require a low-pass filter to make its smoothness, especially on the non-linear function (e.g. sine wave). Left trace: before filter; Right trace: after filter.


I used a Switched Capacitor Filter - MAX264, this active filter is designed for precision filtering applications. The center frequency, Q value, and operating mode (Low/High/Band pass) are all selectable via pin-strapped inputs.

Distortion of the waveform

Distortion occurred in the DDS when a keypad is connected, I spent many time to fix this problem.... >_< , but the reason is too simple, I just forgot to stop the timer interrupt when the waveform is generating.

Components box

Some of my electronics component boxes.

Keypad attached

For easy to input the frequency value and waveform setting, a keypad control should be implement.

DIY a AVR JTAG debugger

A homebrew AVR-JTAG debugger (top right), it includes ICE (in-circuit emulator) and programmer functions.


Top view of the homebrew AVR-JTAG.


Bottom view of the homebrew AVR-JTAG.

Prototype testing

Bread board testing, sine wave generated from the R-2R resistors ladder.


Left: prototype of the homebrew AVR-JTAG debugger; Right: prototype of the DDS.

Wire wrap to pin header

A special connector was made for the AVR-ISP programmer and AVR-JTAG debugger.

2nd hand oscilloscope

I bought a second hand oscilloscope for this project, cost RMB￥400 from ShenZhen electronics market.

Logo for the DDS project