Calibrated Continuous-Time Sigma-Delta Modulators

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CALIBRATED CONTINUOUS-TIME SIGMA-DELTA MODULATORS
A Dissertation
by
CHO-YING LU
Submitted to the Office of Graduate Studies of
Texas A&M University
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
May 2010

ABSTRACT
Calibrated Continuous-Time Sigma-Delta Modulators. (May 2010)
Cho-Ying Lu,
B.A., National Tsing Hua University, Taiwan;
M.S., National Tsing Hua University, Taiwan
Chair of Advisory Committee: Dr. Jose Silva-Martinez
To provide more information mobility, many wireless communication systems such
as WCDMA and EDGE in phone systems, bluetooth and WIMAX in communication
networks have been recently developed. Recent efforts have been made to build the allin-
one next generation device which integrates a large number of wireless services into a
single receiving path in order to raise the competitiveness of the device. Among all the
receiver architectures, the high-IF receiver presents several unique properties for the
next generation receiver by digitalizing the signal at the intermediate frequency around a
few hundred MHz. In this architecture, the modulation/demodulation schemes, protocols,
equalization, etc., are all determined in a software platform that runs in the digital signal
processor (DSP) or FPGA. The specifications for most of front-end building blocks are
relaxed, except the analog-to-digital converter (ADC). The requirements of large
bandwidth, high operational frequency and high resolution make the design of the ADC
very challenging.
Solving the bottleneck associated with the high-IF receiver architecture is a major
focus of many ongoing research efforts. In this work, a 6th-order bandpass continuousiv
time sigma-delta ADC with measured 68.4dB SNDR at 10MHz bandwidth to
accommodate video applications is proposed. Tuned at 200 MHz, the fs/4 architecture
employs an 800 MHz clock frequency. By making use of a unique software-based
calibration scheme together with the tuning properties of the bandpass filters developed
under the umbrella of this project, the ADC performance is optimized automatically to
fulfill all requirements for the high-IF architecture.
In a separate project, other critical design issues for continuous-time sigma-delta
ADCs are addressed, especially the issues related to unit current source mismatches in
multi-level DACs as well as excess loop delays that may cause loop instability. The
reported solutions are revisited to find more efficient architectures. The aforementioned
techniques are used for the design of a 25MHz bandwidth lowpass continuous-time
sigma-delta modulator with time-domain two-step 3-bit quantizer and DAC for WiMAX
applications. The prototype is designed by employing a level-to-pulse-width modulation
(PWM) converter followed by a single-level DAC in the feedback path to translate the
typical digital codes into PWM signals with the proposed pulse arrangement. Therefore,
the non-linearity issue from current source mismatch in multi-level DACs is prevented.
The jitter behavior and timing mismatch issue of the proposed time-based methods are
fully analyzed. The measurement results of a chip prototype achieving 67.7dB peak
SNDR and 78dB SFDR in 25MHz bandwidth properly demonstrate the design concepts
and effectiveness of time-based quantization and feedback.
Both continuous-time sigma-delta ADCs were fabricated in mainstream CMOS
0.18um technologies, which are the most popular in today’s consumer electronics
industry.

TABLE OF CONTENTS
Page
ABSTRACT .............................................................................................................. iii
DEDICATION ........................................................................................................... v
ACKNOWLEDGEMENTS ...................................................................................... vi
TABLE OF CONTENTS .......................................................................................... viii
LIST OF FIGURES ................................................................................................... x
LIST OF TABLES ..................................................................................................... xv
CHAPTER
I INTRODUCTION ............................................................................. 1
1.1 Motivation ................................................................................ 1
1.2 Research Contribution .............................................................. 3
1.3 Dissertation Organization ......................................................... 5
II THE BOTTLENECK OF THE NEXT GENERATION
RECEIVERS: ANALOG-TO-DIGITAL CONVERTER .................. 6
2.1 Next Generation Receivers ....................................................... 6
2.2 Common Receiver Architectures for Wireless
Communication ........................................................................ 9
2.3 Analog-to-Digital Converter (ADC) ........................................ 17
2.4 Conclusion ................................................................................ 24
III OVERSAMPLING ΣΔ ADC ............................................................. 25
3.1 Oversampling ΣΔ ADC ............................................................ 25
3.2 Design Issues of A Continuous-Time ΣΔ ADC ........................ 38
3.3 Design Flow of A ΣΔ ADC ....................................................... 52
3.4 Literature Survey ...................................................................... 55
IV A SELF-CALIBRATED 6TH-ORDER 200MHZ IF BANDPASS
ΣΔ MODULATOR WITH OVER 68DB SNDR IN 10MHZ
BANDWIDTH .................................................................................. 57
4.1 Introduction .............................................................................. 57
ix
CHAPTER . Page
4.2 System Planning ....................................................................... 60
4.3 Circuit Implementation of critical blocks ................................. 70
4.4 Software-Based Calibration ..................................................... 90
4.5 Experimental Results ................................................................ 95
4.6 Conclusion ................................................................................ 103
4.7 Appendix A: Distortion Analysis of The Basic Closed-Loop
System ...................................................................................... 103
4.8 Appendix B: System Simulation in Simulink .......................... 108
V A 25MHZ BANDWIDTH 5TH-ORDER CONTINUOUS-TIME
LOWPASS SIGMA-DELTA MODULATOR WITH 67.7DB SNDR
INTRODUCTION ............................................................................. 110
5.1 Introduction .............................................................................. 110
5.2 System Planning ....................................................................... 112
5.3 Level-to-PWM Converter ........................................................ 117
5.4 Complementary Injection-Locked Frequency Divider ............ 124
5.5 3-Bit Two-Step Current-Mode Quantizer ................................. 124
5.6 Circuit Implementation ............................................................ 128
5.7 Measurement Results ............................................................... 138
5.8 Conclusion ................................................................................ 146
5.9 Appendix: Non-Linearity Analysis of Device Mismatch in
The Proposed PWM Pulses ...................................................... 147
VI CONCLUSION ................................................................................. 151
REFERENCES .......................................................................................................... 153
VITA .......................................................................................................................... 160

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