BASICS OF DIRECT DIGITAL SYNTHESIS Calen Carabajal EECS /3/2013

WHAT DOES A DDS DO?  “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

HOW IT WORKS: THE DDS CORE  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: FREQUENCY TUNING WORD  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: PHASE-TO-AMPLITUDE CONVERSION  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: AN EXAMPLE

FTW EFFECT ON DDS OUTPUT FTW = 0001 FTW = 0010

PHASE AND AMPLITUDE CONTROL  Figure taken from AD9914 datasheet  Direct control of frequency, phase, and amplitude permitted by function pin settings

BINARY PHASE SHIFT KEYING

ON-OFF KEYING (AMPLITUDE SHIFT)

CHIRP GENERATION USING ON-CHIP DIGITAL RAMP GENERATOR

SOURCES  Understanding Direct Digital Synthesis (DDS). paper/5516/enwww.ni.com/white- paper/5516/en  Ask the Application Engineer—33: All About Direct Digital Synthesis. 08/dds.pdf 08/dds.pdf  Fundamentals of Direct Digital Synthesis (DDS) pdf pdf  AD9914 Datasheet. files/data_sheets/AD9914.pdfhttp:// files/data_sheets/AD9914.pdf

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