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Basics of Direct Digital Synthesis

Basics of Direct Digital Synthesis

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Basics of Direct Digital Synthesis

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  1. Basics of Direct Digital Synthesis Calen Carabajal EECS 713 12/3/2013

  2. “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?

  3. 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

  4. 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

  5. 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

  6. 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 How it works: an example

  7. FTW = 0010 FTW = 0001 FTW effect on DDS output

  8. Figure taken from AD9914 datasheet • Direct control of frequency, phase, and amplitude permitted by function pin settings Phase and Amplitude Control

  9. Binary Phase Shift Keying

  10. On-Off Keying (Amplitude Shift)

  11. Chirp Generation using On-chip Digital Ramp Generator

  12. Understanding Direct Digital Synthesis (DDS). www.ni.com/white-paper/5516/en • Ask the Application Engineer—33: All About Direct Digital Synthesis. http://www.analog.com/library/analogdialogue/archives/38-08/dds.pdf • Fundamentals of Direct Digital Synthesis (DDS). http://www.analog.com/static/imported-files/tutorials/MT-085.pdf • AD9914 Datasheet. http://www.analog.com/static/imported-files/data_sheets/AD9914.pdf Sources

  13. Questions?