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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?