Fig 1: Direct digital synthesizer block diagram:

Fig 2 : Phase angle of points on sine wave of DDS:

The synthesizer operates by storing various points in the waveform in digital form and then recalling them to generate the waveform.

Its operation can be explained in more detail by considering the phase advances around a circle as shown in Figure 2.

As the phase advances around the circle this corresponds to advances in the waveform i.e. the greater the number corresponding to the phase, the greater the point is along the waveform.

By successively advancing the number corresponding to the phase it is possible to move further along the waveform cycle.

The digital number representing the phase is held in the phase accumulator.

The number held here corresponds to the phase and is increased at regular intervals.

In this way it can be sent hat the phase accumulator is basically a form of counter.

When it is clocked it adds a preset number to the one already held. When it fills up it resets and starts counting from zero again.

In other words this corresponds to reaching one complete circle on the phase diagram and restarting again

Once the phase has been determined it is necessary to convert this into a digital representation of the waveform. This is accomplished using a waveform map. It is a memory which stores a number corresponding to the voltage required for each value of phase on the waveform. In the case of a synthesizer of this nature it is a sine look up table as a sine wave is required.

In most cases the memory is either a read only memory (ROM) or programmable read only memory (PROM). This contains a vast number of points on the waveform, very many more than are accessed each cycle.

A very large number of points is required so that the phase accumulator can increment by a certain number of points to set the required frequency.

The next stage in the process is to convert the digital numbers coming from the sine look up table into an analogue voltage.

This is achieved using a digital to analogue converter (DAC). This signal is filtered to remove any unwanted signals and amplified to give the required level as necessary