Issue
130 May 2001
DDS-GEN—Part
2: The Generator
Start
•
Direct Digital Synthesis?
• The AD9852 monster chip
• Hardware •
Prototype Construction
• On The Software Side
• Design Methodology
• What’s Next?
• Sources & PDF
Direct
Digital Synthesis?
First I’ll
define the DDS principles. A DDS generator is a purely
digital circuit that can reproduce any periodic waveform
at virtually any frequency, depending on the main clock
frequency and harmonic content of the waveform. As shown
in Figure 1, the DDS generator is simple.
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(Click
here to enlarge)
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Figure
1—At each clock pulse, a fixed value (55°
in this example) is added to a phase accumulation
register. The resulting phase is then converted
to amplitude with a sinus look-up table and ADC.
|
At each clock
tick, a fixed amount is added to a phase accumulator register,
modulo 360°. The resulting phase is used as an index for
a look-up table that gives the signal amplitude for each
phase value. Then, the amplitude is converted to an analog
value by an A/D converter and low-pass filtered to exclude
any harmonic above the Nyquist limit. When used to generate
a sinus signal, a DDS generator with N-bits long phase
registers can generate any frequency from:
to
with a resolution
of
where Fclock
is the frequency of the main clock.
With DDS technology,
because everything is digital, it’s easy to implement
complex modulation schemes as frequency shift keying or
phase shift keying. For the former, swap between two phase
increment values, and for the latter, just add a fixed
value to the phase. The difficulties start when you need
a high main clock frequency.
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