Based on the above, because of limits of the reconstruction filter a 300MSa/s D/A can only output a 120MHz maximum frequency. But a 1.2GSa/s D/A can achievea higher upper frequency limit.Of course in the interpolated DDS structure, the digital filter in the interpolator will restrict the frequency to the Nyquist limit of the DDS clock (e.g. 150MHz), but a digital filter is much easier to design than an analog reconstruction filter. Using the interpolated DDS,it is easy to raise the upper limit of output frequency from 120MHz to 130MHz or more.

4.2 To Remove the Spurs Generated by D/A Convertor
Intermodulation distortion isunavoidable in a D/A convertor. In a traditional DDS structure it is difficult to remove some inter-modulation distortion components between the clock and the output signal, such as forumla2 The figure below shows an example with 120MHz output frequency and 300MSa/s sampling rate. The distortion fs – 2fout= 60MHz falls into the pass band of the reconstruction filter. It is not possible to be removed.

Figure 8

But with a 4X interpolated DDS structure, for 120MHz frequency output, the distortion components = 960MHz, =840MHz, which are located far away from pass band of the reconstruction filter.

5 Summary
Compared with a traditional DDS design, the Interpolated DDS technique inserts an interpolator between the DDS circuitry and the D/A convertor, yielding a higher sampling rate. With such a structure the design of the reconstruction filter is no longer the bottleneck for bandwidth. On the other hand, the problems due to intermodulation distortion products falling into the pass band of the reconstruction filter is solved, resulting in better SFDR performance.

6 Reference
[1] Analog Devices, A Technical Tutorialon Digital Signal Synthesis
[2] A.V. Oppenheim, R.W. Schafer, J.R. Buck, Discrete-time Signal Processing


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