A DDS uses a digital control word to set the frequency of its sinusoidal output. Analog Devices refers to the digital control word as the Frequency Tuning Word or FTW. Because a DDS has a sampling clock, namely its system clock, it is by definition a Nyquist device. Therefore, the sinusoidal output frequency of the DDS, as defined by the FTW, spans a range from DC to one half of the system clock frequency. Furthermore, the output spectrum of a DDS contains the image frequencies normally associated with a sampled system.