First what do we mean by DDS – DDS stands for direct digital synthesis. A DDS chip generates a waveform from an internal ROM with table. The ROM table could contain square, triangular, sawtooth or sine wave forms. A high frequency oscillator is used to drive a DDS chip – the high frequency clock is taken into a large internal divider that in turn would generate clock signals of less than 1Hz. The derived low frequency signal will be used to tell the on-chip digital to analog converter to go to the next value on the ROM table.
This particular project makes use of the AD9833 DDS sinewave generator from analog devices. A 50MHz clock was used to drive the AD9833 while an ATmega32 was used as the main controller that takes user input though a numeric keypad, displays the current output through a 16×2 liquid crystal display (LCD) and communicates with the AD9833 through 3 wire SPI. The challenge in building the project comes with the fact that high speed waveforms are difficult to amplify and easily gets infested with noise. An LM7171 was used to amplify the 400mV signal to +-15V, the design also includes proper placement of components such as decoupling capacitors that needs to be very close to the integrated circuits.