1. EE521 Analog and Digital Communications
James K. Beard, Ph. D.
Tuesday, January 25, 2005
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2. Essentials Text: Bernard Sklar, Digital Communications, Second Edition
Prerequisites
Consent of Instructor (Dr. Silage)
SystemView (CD-ROM with text)
Office
E&A 709
Hours TBA, Tuesday afternoons planned
MWF 10:30 AM to 11:30 AM added
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3. TU Blackboard Temple University Home Page Click on TU Portal link
Click on TU Portal link
Log on
TU Blackboard courses will appear
Click on EE521
11/18/2018
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4. TU Blackboard Features
Announcements
Course Information
Staff Information
Course Documents
Assignments
Communication
Discussion Board
External Links
Tools
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5. Today’s Topics SystemView Signals and Noise
Detection of Binary Signals
Inter-symbol interference
Equalization
Discussion (as time permits)
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6. Formatting and Baseband Modulation
Topics from Text Chapter 2
2.5, Sources of Corruption
2.6, Pulse Code Modulation
2.7, Uniform and Non-uniform Quantization
2.8, Baseband Modulation
Corruption: noise, fading, interference
PCM: Simple digital modulation
Quantization: the ADC and data compression
Baseband modulation: synthesizing the signal
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7. Sklar Chapter 2 Legend: From other sources Essential Information
Message symbols
Optional
Channel symbols
X M I T
Format
Source encode
Encrypt
Channel encode
Multi-plex
Pulse modulate
Bandpass modulate
Freq-uency spread
Multiple access
Digital input
Channel impulse response
Bit stream
Synch-ronization
Digital baseband waveform
Digital bandpass waveform
Channel
Digital output
R C V
Format
Source decode
Decrypt
Channel decode
Demul-tiplex
Detect
Demod-ulate & Sample
Freq-uency despread
Multiple access
To other destinations
Channel symbols
Information
sink
Message symbols
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8. Message, Characters and Symbols
Definitions
Message – the data to be presented to the data sink or used to generate analog output at the receiver
Character – base message unit, such as a letter of the alphabet or an 8 or 16 bit word produced by an ADC
Symbol – a grouping of bits for encoding
Characters and symbols are often different sizes
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9. Formatting Analog Information
Operations are
Sampling – uniformly spaced captures of a waveform
Digitization – reduction of a data sample to a set of discrete levels
Limitations include
Aliasing; the digitized forms of two signal differing from one another by the sample rate are identical
Quantization; may be uniform or non-uniform, and presents a noise floor
Clipping; there is a maximum value that may be represented
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10. Sampling Can be characterized as An ADC operates on the held signal
Switch a waveform into a follower
At the sample time
Switch a holding capacitor into the holding circuit
Switch off the waveform input
The follower becomes an integrator with no input and holds the signal
An ADC operates on the held signal
Sampling is an analog operation and has a frequency response
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11. Aliasing
Definition:
A tone signal with a frequency higher than the sample rate will produce a data format identical to that of a tone with a frequency lower than sample frequency
The difference between the two frequencies is an integer multiple of the sample frequency
Usual ambiguity range is from minus half the sample frequency to plus half the sample frequency
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12. Issues With Sampling
Signal center frequency is often much larger than the bandwidth
Two techniques are used to keep data rate to twice the signal bandwidth
Quadrature demodulation, the use of two LO’s in quadrature with two mixers to produce two baseband signals in quadrature
Digital quadrature demodulation, undersampling at IF followed by digital quadrature demodulation
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13. Real vs. Complex Sampling
For real sampling at frequency fs > 2.B
The ambiguity range is 0 to fs
Negative frequencies are ambiguous with positive frequencies
For quadrature demodulation and output samples at frequency fqs > B
The ambiguity range is –fqs/2 to +fqs/2
Positive and negative frequencies are unambiguous
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14. Real and Complex Sampling
Negative frequencies
Distinct with complex samples
Ambiguous with real samples
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15. Generating a Passband Signal
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16. Quadrature Demodulator
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17. Digital Quadrature Demodulation
Undersample
Signal at frequency f0 with bandwidth B
Sample at rate fqs
Result
Signal aliases to plus or minus half the sample rate
FIR filter and decimate to produce the final result
Signal is then ready for FFT or other processing
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18. Digitization Levels Uniform and non-uniform commonly used
Uniform sampling levels – used for high quality systems
Requires more bits, typically 10 to 16 bits
Music
Radar and sonar
Non-uniform sampling levels – used where bandwidth drives the channel utility
Cell phones
Some desk phones and VOIP
Typically 8 bits
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19. Effects Both types have a “hard” ceiling
Extra ADC circuitry offers hard limiting in place of end-around overflow
Noise floor is different
Uniform levels – usually characterized as an additive noise with an RMS value of 1/12 the LSB (see next slide for real-world considerations)
Nonuniform levels – COMDAC levels or fine at low amplitudes, coarse at high levels
Quantization noise floor higher for large signals
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20. Real-World Sampling and ADC
Sampling is
The last mixer in the receiver chain
Produces signal at baseband from signal at IF
Sample clock phase noise is part of the signal chain error sources
Dynamic range
Dynamic range is less than theoretical with nearly all ADCs
Effective number of bits (ENOB) and SNR are two synonymous ways of specifying ADC dynamic range
ENOB is typically 1.5 bits less than the word length of an ADC
Spurious spectral lines
Sample aperture time and duration jitter can cause low level tonal components in ADC output
Can come from inside the ADC chip or from the sample clock
Some resulting spurious lines are signal-dependent
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21. Trades in Quadrature Demodulation
Trades for analog quadrature demodulator
Matching of channels for in-phase and quadrature channels limits negative frequency discrimination
Negative frequency ghosting typical 40 dB to 60 dB
Traces for digital quadrature demodulator
Ghosting is determined by sampling and FIR filtering/decimation scheme
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22. Why Digital Quadrature Demodulation
Advantages
Only one ADC, no multiplexing or matching
Performance is determined by
Antialiasing filtering prior to ADC
Specifications of FIR decimation filters
Disadvantages
Sampling is done at IF
Jitter and aperture time specs are tougher
Exchanged difficulties – from analog channel matching to problems in sampling at IF
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23. SystemView Overview
Simulations of digital signal processing operations
Analog, through oversampling and emulation of analog operations
Digital, through direct emulation
Analysis
Bode plots, root locus, other
Visualization
Time domain plots
Virtual spectrum analyzer
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24. Why SystemView
Dual-gate MOSFET RF converter, 144 MHz to 10 MHz. Uses the RCA 3N140 for its overload and mixing capabilities. Circa 1968.
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25. 11/18/2018
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26. Summary
Course overview: we will learn how to implement critical technologies for digital communications
Course is integrated with a follow-on next semester
Baseband signals are where we do modulation == formatting for the channel
SystemView lets us have most of the benefits of a lab with minimum time
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27. Feedback Around the room Please let me know What your background is
Design
SystemView
Communications
What you expect from this course
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28. Text and Assignment Text Assignment: Read Text
Benard Sklar, Digital Communicatinons ISBN
SystemView User's Manual, Elanix, Inc
Assignment: Read Text
Chapter 2, 2.1, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8
Load SystemView and examine the samples and demos
Browse appendices of text for review and supplementary material
Look at TUARC
K3TU, websites
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