Direct digital synthesis

Direct digital synthesis (DDS) is a method employed by frequency synthesizers used for creating arbitrary waveforms from a single, fixed-frequency reference clock.

The reference oscillator provides a stable time base for the system and determines the frequency accuracy of the DDS.

The output reconstruction filter rejects the spectral replicas produced by the zero-order hold inherent in the analog conversion process.

In order to reject these undesired images, a DDS is generally used in conjunction with an analog reconstruction lowpass filter as shown in Figure 1.

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.

[8] The superior close-in phase noise performance of a DDS stems from the fact that it is a feed-forward system.