BASICS OF DIRECT DIGITAL
SYNTHESIS
Calen Carabajal
EECS 713
12/3/2013

“Direct Digital Synthesis is a method of
producing an analog waveform—
usually a sine wave—by generating a
time-varying signal in digital form and
then performing a digital-to-analog
conversion. [1]”

Capable of generating arbitrary
waveforms and accomplishing
various modulation schemes.

High-speed DDS applications include,
primarily, radar and communication
systems
WHAT DOES A DDS DO?

The core of the DDS is the phase
accumulator and a lookup table,
which comprise a Numerically
Controlled Oscillator

N-bit word in frequency control
register (Frequency Tuning Word, FTW)
is input to adder, added to feedback
accumulator output.

Truncated output serves as
address/input for sin() lookup table.

Output word is amplitude for given
phase value. Converted to analog.
HOW IT WORKS: THE DDS CORE

The FTW determines the jump size on
a phase wheel.

Each clock cycle, the phase
accumulator is updated by the
amount given by the frequency
tuning word.


Output of phase accumulator is a
ramp function with slope FTW
When accumulator surpasses 2^n, it
cycles back to beginning
HOW IT WORKS: FREQUENCY TUNING
WORD

Phase-to-amplitude conversion is a
block of memory.

Contains values ~2^M values of sine
wave amplitude


Only needs to contain first 90
degrees, not 360 degrees.
Accumulator output bits serve as
address
HOW IT WORKS: PHASE-TO-AMPLITUDE
CONVERSION

Basic example: single-tone
operation

Using a N=4 bit DDS

FTW: 0001

Phase accumulator ramps up 1 LSB
each clock cycle

DDS output is sin 2𝜋 ∗
𝑃𝐴𝑜𝑢𝑡
2𝑁
HOW IT WORKS: AN EXAMPLE
FTW EFFECT ON DDS OUTPUT

Figure taken from AD9914
datasheet

Direct control of frequency, phase,
and amplitude permitted by
function pin settings
PHASE AND AMPLITUDE CONTROL
BINARY PHASE SHIFT KEYING
ON-OFF KEYING (AMPLITUDE SHIFT)
CHIRP GENERATION USING ON-CHIP
DIGITAL RAMP GENERATOR

Understanding Direct Digital Synthesis (DDS). www.ni.com/whitepaper/5516/en

Ask the Application Engineer—33: All About Direct Digital
Synthesis.
http://www.analog.com/library/analogdialogue/archives/3808/dds.pdf

Fundamentals of Direct Digital Synthesis (DDS).
http://www.analog.com/static/imported-files/tutorials/MT085.pdf

AD9914 Datasheet. http://www.analog.com/static/importedfiles/data_sheets/AD9914.pdf
SOURCES
QUESTIONS?