Direct Digital
Synthesis
(continued)
A direct digital synthesis (DDS) signal generator differs from a digital
audio player because of its very precise control of the data stream input
to the
DAC. In a DDS system, the amplitude values for one complete
cycle of the output waveshape are stored sequentially in random access
memory (RAM) as shown in the figure below. As
RAM addresses are
changed, the
DAC converts the waveshape data into a voltage waveform
(whose data values are loaded in
RAM). The frequency of the voltage
waveform is proportional to the rate at which the
RAM addresses
are changed.
The 33120A represents amplitude values by 4,096 discrete voltage
levels (or 12-bit vertical resolution). Waveforms may contain between
8 points and 16,000 points of 12-bit amplitude values. The number of
points in
RAM representing one complete cycle of the waveshape
(or 360
) is called its length or horizontal resolution. Each RAM address
corresponds to a phase increment equal to 360
/ points, where points is
the waveform length. Therefore, sequential
RAM addresses contain the
amplitude values for the individual points (0
to 360) of the waveform.
4096
2047 DAC Codes
0
0
90
180
270
360
0 3,999 7,999 11,999 15,999
Memory Address (Points)
Chapter 7 Tutorial
Direct Digital Synthesis
274
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