Low cost signal generator

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POLITECNICO DI TORINO
School of Information Engineering
Graduation in Electronic and Computer Engineering

Bachelor of Science Thesis

Design and Realization of a low cost Signal Generator by means of Direct Digital Synthesis

Supervisor: Prof. Marco Parvis Eng. Simone Corbellini

Candidate: Rodrigo Alves da Silveira

July 2010

To my family

Introduction

IntroductionElectronics in the past few decades have evolved in a way that mankind could never imagine, making it possible to achieve technologies that would completely re-design their way of life. With each new discovery, it comes an uncountable number of new possibilities. That is the way Electronics works, always being reinvented in order to attend the needs of the modern society. The ability to accuratelyproduce and control waveforms of various frequencies and profiles has become a key requirement common to a number of industries. Whether providing agile sources of low-phase-noise variable-frequencies with good performance for communications or simply generating a frequency stimulus in industrial or biomedical equipment. That is when it comes the DDS (Direct Digital Synthesis). DDS is a method ofproducing an analog waveform (usually a sinusoidal wave) by generating a time variant signal in digital form and then performing a digital-to-analog conversion. It provides remarkable frequency resolution and allows direct implementation of frequency, phase and amplitude modulation. With advances in design and process technology, today’s DDS devices are very compact and draw little power. That is whyit was chosen to implement the project behind this thesis, the design of a low-cost signal generator, by means a microcontrolled DDS.

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TABLE OF CONTENTS

TABLE OF CONTENTS
1 The Signal Generator Project 1

2 The Direct Digital Synthesis 2.1 2.2 2.3 The reasons for using DDS The main kinds of output in a typical device The generation of the sine wave 2.3.1 2.3.2 2.3.3 2.3.4 The phaseaccumulator The phase-to-amplitude converter The digital-to-analog converter The low-pass filter

3 3 3 4 5 7 7 8

3 The AD9859 DDS 3.1 3.2 3.3 3.4 3.5 The requested alimentation Determining the synthesized output frequency The internal registers of AD9859 The input clock The output characteristics

10 14 14 15 16 16

4 The Designed System 4.1 The control unit 4.1.1 4.1.2 4.1.3 IntroductionThe microcontroller The PIC32MX440F256H microcontroller

18 19 19 20 20 iii

TABLE OF CONTENTS

4.1.4 4.1.5 4.1.6 4.1.7 4.1.8 4.2

The clock The CPU The input/ output pins The communication interface The RS-232 interface

25 25 26 26 28 28 28 32 32 34 34 36 38

The TUSB 4.2.1 The TUSB3410

4.3

The reference resistance RDAC 4.3.1 The digital potentiometer AD5231 Output stage4.4.1 4.4.2 4.4.3 The digital-to-analog converter DAC8501 The amplification stage The filtered output

4.4

5 The Power Supply Section 5.1 5.2 The transformer The voltage regulators 5.2.1 5.2.1 The LFxxABT The UCC384DP

40 41 41 42 42

A Electrical Scheme A.1 The Circuit Schematic A.2 The PCB

45 45 48

Conclusion

51

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Chapter 1 – The Signal Generator Project

Chapter 1 TheSignal Generator Project
The purpose of this project is to create a portable continuous wave signal generator for general use. This device will be able to replace the expensive and large signal generators commonly used in laboratories, for those applications where high quality signals are not required. Signal or function generators have been around for a long while. Over time, these instrumentshave accumulated a long list of features. Starting with just a few knobs for setting the amplitude and frequency of a sinusoidal output, function generators now provide wider frequency ranges, calibrated output levels, a variety of waveforms, modulation modes, computer interfaces, and in some cases, arbitrary functions. The many features added to signal generators have complicated their design and...
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