thumb|upright=1.25|A DDS [[function generator]]

Direct digital synthesis (DDS) is a method employed by frequency synthesizers to create arbitrary waveforms from a single, fixed-frequency reference clock. DDS is used in applications such as signal generation, local oscillators in communication systems, function generators, mixers, modulators, as shown in Figure 1.

The reference oscillator 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.

Performance

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. Disadvantages include spurious responses mainly due to truncation effects in the NCO, crossing spurs resulting from high order (>1) Nyquist images, and a higher noise floor at large frequency offsets due mainly to the digital-to-analog converter.

Frequency agility

The output frequency of a DDS is determined by the value stored in the frequency control register (FCR) (see Fig.1), which in turn controls the NCO's phase accumulator step size. Because the NCO operates in the discrete-time domain, it changes frequency instantaneously at the clock edge coincident with a change in the value stored in the FCR. The DDS output frequency settling time is determined mainly by the phase response of the reconstruction filter. An ideal reconstruction filter with a linear phase response (meaning the output is simply a delayed version of the input signal) would allow instantaneous frequency response at its output because a linear system can not create frequencies not present at its input.

</references>

  • Tutorial on Digital Signal Synthesis (From Analog Devices)
  • L. Cordesses, "Direct Digital Synthesis: A Tool for Periodic Wave Generation (Part 1)" IEEE Signal Processing Magazine, DSP Tips & Tricks column, pp.&nbsp;50–54, Vol. 21, No. 4 July 2004.
  • L. Cordesses, Direct Digital Synthesis: A Tool for Periodic Wave Generation (Part 2) IEEE Signal Processing Magazine, DSP Tips & Tricks column, pp.&nbsp;110–117, Vol. 21, No. 5, Sep. 2004.