Direct digital synthesis (DDS) is a digital method of producing an analog waveform using clock signal and digital-to-analog conversion. Direct Digital Synthesizer is a type of frequency synthesizer used for creating arbitrary waveforms from a single fixed-frequency reference clock signal. Applications of DDS include: signal generation, local oscillators in communication systems, function generators, frequency mixers, modulators, sound synthesizers and as part of a digital phase-locked loop. DDS devices are not limited to purely sinusoidal outputs. It is possible to generate Square-, triangular-, sawtooth, sinusoidal-waves, etc.

Direct digital synthesis (DDS) is a method of producing an analog waveform—usually a sine wave—by generating a time-varying signal in digital form and then performing a digital-to-analog conversion. Because operations within a DDS device are primarily digital, it can offer fast switching between output frequencies, fine frequency resolution, and operation over a broad spectrum of frequencies. With advances in design and process technology, today’s DDS devices are very compact and draw little power.

Operating Principle of Direct Digital Synthesizer

Direct Digital SynthesizerA basic Direct Digital Synthesizer consists of a frequency reference (often a crystal or SAW oscillator), a numerically-controlled oscillator and a digital-to-analog converter (DAC). The reference provides a stable time base for the system and determines the frequency accuracy of the DDS. It provides the clock to the NCO which produces at its output a discrete-time, quantized version of the desired output waveform (often a sinusoid) whose period is controlled by the digital word contained in the Frequency Control Register. The sampled, digital waveform is converted to an analog waveform by the DAC. The output reconstruction filter rejects the spectral replicas produced by the zero-order hold inherent in the analog conversion process.

Advantages Direct Digital Synthesizer

A DDS has many advantages over its analog counterpart, the phase-locked loop (PLL), including much better frequency agility, improved phase noise, and precise control of the output phase across frequency switching transitions. DDS provides remarkable frequency resolution and allows direct implementation of frequency, phase and amplitude modulation. Today’s cost-competitive, high-performance, functionally integrated DDS ICs are becoming common in both communication systems and sensor applications. The advantages that make them attractive to design engineers include:

  • Digitally controlled micro-hertz frequency-tuning and sub-degree phase-tuning capability,
  • Extremely fast hopping speed in tuning output frequency (or phase); phase-continuous frequency hops with no overshoot/undershoot or analog-related loop settling-time anomalies,
  • The digital architecture of DDS eliminates the need for the manual tuning and tweaking related to component aging and temperature drift in analog synthesizer solutions, and
  • The digital control interface of the DDS architecture facilitates an environment where systems can be remotely controlled and optimized with high resolution under processor control.