This AVR DDS generator can generate signals from 1Hz up to 65535Hz. Thanks to well known DDS algorithm, frequency limits can be much higher, but signal resolution drops in higher frequencies. The current range is enough for most audio signal equipment testing. DDS algorithm is written in software where Atmega8 runs at a maximum 16MHz. There are several signal waveforms available: sine, square, sawtooth, triangle, reverse sawtooth and pseudo-random noise.
Signals are stored in lookup tables as the DDS algorithm requires. Signal output is organized via simple R-2R ladder DAC. LCD signal goes through the shift register; this way, only 3 MCU pins are used, while others serve for other purposes – buttons other outputs. The project is well documented PCB and source files included. The author states that he will build another DDS generator with improved control and variable signal amplifier at the output.