Direct Digital Synthesis DDS

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2. Overview. Introduction to Direct Digital SynthesisApproaches to DDSPulse output DDSROM lookup tableImpulse response of a filter. 3. Overview. Advanced techniques: Bandpass signal generationSources of spurious signals and their effectsTechniques used to minimize spurious signalsGeneration o

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Direct Digital Synthesis DDS

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1. Direct Digital Synthesis (DDS) Dr. Jeffrey H. Reed Mobile and Portable Radio Research Group (MPRG) Virginia Tech Bradley Dept. of Electrical and Computer Engineering [email protected]

2. 2 Overview Introduction to Direct Digital Synthesis Approaches to DDS Pulse output DDS ROM lookup table Impulse response of a filter

3. 3 Overview Advanced techniques: Bandpass signal generation Sources of spurious signals and their effects Techniques used to minimize spurious signals Generation of Random Sequences Summary and Future Trends

4. 4 Introduction to DDS Direct digital synthesis (DDS) is the process of generating deterministic communication carrier/reference signals directly in discrete time with the use of digital hardware Discrete time signals are then converted into analog signals (for transmission) using a D/A converter

5. 5 Need for DDS systems Overcome the limitations of analog synthesis Speed, precision, size, flexibility, stability, and ease of implementation Compatible with and desirable for today’s high speed digital communication technology

6. 6 Early DDS Systems First DDS designs date back to the early 70s Tierney et. al. developed a technique for generating audio signals Used a Read Only Memory (ROM) to store sine waves Stored values were used to drive a D/A followed by analog interpolation filter

7. 7 Early DDS Systems Roke Manor laboratories in 1981 of the then Plessey company’s prototype DDS Occupied several complete boards of logic laid out on the bench Clocked at 10MHz Output frequency of up to 3MHz Spurious responses about 40 dB below the desired output

8. 8 Modern DDS Systems Gained importance in the early 80’s with the widespread use of digital communication systems Have incorporated a lot of changes and improvements making them a practical alternative to analog signal sources GHz frequencies possible, spurs of -60 to -80 dB or lower

9. 9 Analog Generation Techniques Direct Analog Synthesis (DAS) Generate frequencies by mixing frequencies from different crystal and/or using their harmonics Ideal situation with tuning capabilities of LC oscillator and stability and purity of a crystal oscillator

10. 10 Characteristics Advantages High purity, low spurious content: better than -80 dB Fast switching: .1 - 20 ?s Disadvantages Bulky, expensive, high power consumption Not suitable for portable equipment Used in medical and radar imaging, spectroscopy and frequency hopping systems

11. 11 Analog Generation Techniques

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