Transcript Slide 1

EE521 Analog and
Digital
Communications
James K. Beard, Ph. D.
[email protected]
Tuesday, January 25, 2005
http://astro.temple.edu/~jkbeard/
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Essentials
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Text: Bernard Sklar, Digital Communications,
Second Edition
Prerequisites
 Consent
of Instructor (Dr. Silage)
 SystemView (CD-ROM with text)
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Office
 E&A 709
 Hours
TBA, Tuesday afternoons planned
 MWF 10:30 AM to 11:30 AM added
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TU Blackboard
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Temple University Home Page
 http://www.temple.edu/
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Click on TU Portal link
 http://tuportal.temple.edu/
Log on
 TU Blackboard courses will appear
 Click on EE521
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TU Blackboard Features
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Announcements
Announcements
Course Information
Staff Information
Course Documents
Assignments
Communication
Discussion Board
External Links
Tools
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Today’s Topics
SystemView
 Signals and Noise
 Detection of Binary Signals
 Inter-symbol interference
 Equalization
 Discussion (as time permits)
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Formatting and Baseband
Modulation
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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
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Corruption: noise, fading, interference
PCM: Simple digital modulation
Quantization: the ADC and data compression
Baseband modulation: synthesizing the signal
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Sklar Chapter 2
From
other
sources
Information
source
Message
symbols
Source
encode
Format
Essential
Legend:
Optional
Channel
symbols
Encrypt
Channel
encode
Multiplex
Pulse
modulate
gi  t 
ui
Digital
input
mi
Bit
stream
z T 
uˆi
Format
Information
sink
Message
symbols
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Decrypt
Channel
decode
Demultiplex
si  t 
Digital
baseband
waveform
Synchronization
Digital
output
mˆ i
Source
decode
Bandpass
modulate
Detect
Frequency
spread
Digital
bandpass
waveform
X
M
IT
Multiple
access
hc  t 
Channel Channel
impulse
response
r t 
Demodulate &
Sample
Frequency
despread
R
C
V
Multiple
access
To other Channel
destinations symbols
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Message, Characters and
Symbols
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Definitions
– 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
 Message
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Characters and symbols are often
different sizes
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Formatting Analog
Information
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Operations are
 Sampling – uniformly spaced captures of a
 Digitization – reduction of a data sample to
waveform
a set of
discrete levels
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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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Sampling
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Can be characterized as
 Switch
a waveform into a follower
 At the sample time
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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
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An ADC operates on the held signal
Sampling is an analog operation and has a
frequency response
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Aliasing
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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
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Usual ambiguity range is from minus half the
sample frequency to plus half the sample
frequency
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Issues With Sampling
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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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Real vs. Complex Sampling
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For real sampling at frequency fs > 2.B
 The
ambiguity range is 0 to fs
 Negative frequencies are ambiguous with
positive frequencies
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For quadrature demodulation and output
samples at frequency fqs > B
ambiguity range is –fqs/2 to +fqs/2
 Positive and negative frequencies are
unambiguous
 The
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Real and Complex Sampling
Negative frequencies
Distinct with complex samples
Ambiguous with real samples
fS

2
0
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fS
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2
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Generating a Passband Signal
Signal I
Noise I
Signal Q
Noise Q
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
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
 cos   t 
1
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 sin 1  t 
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



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Passband
Signal
15
Quadrature Demodulator
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Passband
Signal
cos 1  t 
sin 1  t 
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Signal I
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Signal Q
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Digital Quadrature
Demodulation
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Undersample
 Signal
at frequency f0 with bandwidth B
 Sample at rate fqs
4 f0
f qs 
B
2k  1
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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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Digitization Levels
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Uniform and non-uniform commonly used
Uniform sampling levels – used for high quality
systems
 Requires
 Music
 Radar
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more bits, typically 10 to 16 bits
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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Effects
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Both types have a “hard” ceiling
Extra ADC circuitry offers hard limiting in place
of end-around overflow
Noise floor is different
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
 Uniform
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Real-World
Sampling and ADC
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Sampling is
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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
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Dynamic range
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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
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Spurious spectral lines
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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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Trades in Quadrature
Demodulation
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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
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Traces for digital quadrature demodulator
 Ghosting
is determined by sampling and FIR
filtering/decimation scheme
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Why Digital Quadrature
Demodulation
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Advantages
 Only
one ADC, no multiplexing or matching
 Performance is determined by
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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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SystemView Overview
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Simulations of digital signal processing
operations
 Analog,
through oversampling and emulation of
analog operations
 Digital, through direct emulation
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Analysis
 Bode
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plots, root locus, other
Visualization
 Time
domain plots
 Virtual spectrum analyzer
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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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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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Feedback
Around the room
 Please let me know
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 What
your background is
EE
 Design
 SystemView
 Communications
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 What
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you expect from this course
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Text and Assignment
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Text
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Benard Sklar, Digital Communicatinons ISBN 0-13-084788-7
SystemView User's Manual, Elanix, Inc
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Assignment: Read Text
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http://www.elanix.com/
http://www.elanix.com/pdf/SVUGuide.pdf
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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7/16/2015
http://www.temple.edu/ece/tuarc.htm
http://www.temple.edu/k3tu
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