6.10: Direct Digital Synthesizer

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A direct digital synthesizer (DDS) uses a DSP unit, commonly called a numerically controlled oscillator, to create the digital version of a modulated waveform (see Figure 6.9.4). The digital output of the DSP drives a DAC that is filtered by a lowpass filter producing the analog output. A DDS module enables a fairly complex RF system to be realized using just a few components, as frequency generation and frequency synthesis are embodied in one unit. The accuracy of the frequency of the analog signal is established by the reference oscillator that acts as a precision clock for the DSP and DAC.

The numerically controlled oscillator has an accumulator, often called a phase accumulator, that counts out the phase of the analog signal to be synthesized. If there is modulation, then it is incorporated at this stage. Both modulation and the center frequency of the modulated output are changed by changing the accumulator’s step size. The accumulator drives a sine lookup table that then produces the digital words presented to the DAC.

A DDS has the advantage that the output frequency can be changed almost instantaneously which is in contrast to a PLL in which the rate of frequency change of the output is limited by the bandwidth of the loop filter. A major disadvantage of a DDS is the number of spurious signals generated by quantization issues, and this is mostly related to the bit length of the accumulator. There are also aliasing or image signals generated as images of the intended signal relative to the clock frequency. However, the spurious signals from two DDS units with the same reference oscillator are correlated and this can be exploited in some system architectures to remove the effect of spurious signals. Most of the negative characteristics of a DDS are reduced by using a longer accumulator and hence a DAC with a larger number of input bits. This, however, leads to increased power consumption. These drawbacks are sometimes insignificant when compared to the frequency agility and system simplicity achieved.