Fluke DDS Function Generator 271, User Manual, Page: 115 / 132

Appendix B
DDS Operation and Further Waveform
Considerations
Introduction
This appendix gives some further information on DDS operation as a background to
understanding both the advantages and the limitations of DDS waveform generation.
DDS Operation
10 Bit 10 Bit
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One complete cycle of the selected waveform is stored in RAM as 1024 10-bit amplitude
values. As the RAM address is incremented the waveform values are output sequentially
to a digital-to-analogue converter (DAC) which reconstructs the waveform as a series of
voltage steps. Sine waves and triangle waves are subsequently filtered to smooth the steps
in the DAC output.
The frequency of the output waveform is determined by the rate at which the RAM
addresses are changed; in a DDS system the address changes are generated as follows.
The RAM contains the amplitude values of all the individual points of 1 cycle (360 °) of
the waveform; each sequential address change corresponds to a phase increment of the
waveform of 360 ° divided by 1024. Instead of using a counter to generate sequential
RAM addresses, a phase accumulator is used to increment the phase.
B-1
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