Direct Digital Synthesis

Abstract

Direct Digital Synthesizer (DDS) is the more advanced function generator which can solve the problems of conventional function generators to a great extent. It has an Increasing role in digital clock and agile clock generation, and modulation. DDS roots from sampling and quantizing theorems and generate wide range of frequencies with distortion less than-55dB below 50 kHz and less than-40dB above 50 kHz. Also DDS can support Amplitude modulation, frequency modulation, or phase modulation. DDS uses a fixed-frequency clock and a simpler filtering scheme, so it's less expensive than the Points per Clock (PPC) method. In DDS, a phase accumulator a DDS an increment to its output in every clock cycle, and the accumulator's output represents the phase of the waveform. The output frequency is proportional to the increment, so it's easy to change frequency even though the clock frequency is fixed. The output of the accumulator is converted from phase data into amplitude data typically by passing it through some type of look-up table. The phase accumulator design allows DDS to use a fixed clock, but still execute waveforms at a perceived faster sample rate than the clock. With DDS, not every individual point in the waveform memory is being expressed in the resulting output waveform. Instead, DDS outputs a best approximation of the waveform, which means small features in the waveform can be partially or completely skipped over.