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. Current range is enough for most of audio signal equipment testing. DDS algorithm is written in software where Atmega8 runs at maximum 16MHz. There are several signal waveforms available: sine, square, sawtooth, triangle and reverse sawtooth and pseudo-random noise.
Signals are stored in lookup tables as DDS algorithm requires. Signal output is organized via simple R-2R ladder DAC. LCD signal goes through shift register, this way only 3 MCU pins are used, while other serves for other purposes – buttons, other outputs. Project is well documented PCB and source files included. Author states that he will build another DDS generator with improved control and variable signal amplifier at the output.