Outline

1
Outline

• Analog-to-Digital Conversion - Sampling
• Digital Modulation Schemes

• Revisit Analog Modulation Schemes
• Amplitude Modulation (AM)
• Frequency Modulation (FM)

2
Modulation Process

• Information-bearing signals (e.g., voice, video)
  are called baseband signals. Other terms for
  information-bearing signals are message signal
  and modulating wave.
• Modulation is defined as the process by which
  some characteristics of a carrier signal
  (typically a cosine wave) is varied in accordance
  with a message signal.
• Modulation process is required to shift the
  frequency content of our message signals to a
  range that is acceptable by the transmission
  medium. (e.g., above 30 KHz for wireless
  transmission).

3
Modulation Types
Analog Modulation
Digital Modulation
Message signal is analog (a.k.a continuous-time).
Message signal is digital (a.k.a discrete-time).

• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)

• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)

4
Digital Modulation Schemes
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Figure 4-8
Amplitude Change
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Figure 4-9
Frequency Change
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Figure 4-10
Phase Change
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Figure 5-24
Amplitude Shift Keying
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Figure 5-27
Frequency Shift Keying
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Figure 5-29
Phase Shift Keying
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Figure 5-30
PSK Constellation
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Figure 5-31
Quadrature PSK - QPSK 4-PSK
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Figure 5-32
QPSK Constellation
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Figure 5-33
8-PSK Constellation
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Figure 5-35
4-QAM and 8-QAM Constellations
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Figure 5-36
8-QAM Signal
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Figure 5-37
16-QAM Constellation
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Figure 5.17 Bit and baud
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Table 5.1 Bit and baud rate comparison
Modulation Units Bits/Baud Baud rate Bit Rate
ASK, FSK, 2-PSK Bit 1 N N
4-PSK, 4-QAM Dibit 2 N 2N
8-PSK, 8-QAM Tribit 3 N 3N
16-QAM Quadbit 4 N 4N
32-QAM Pentabit 5 N 5N
64-QAM Hexabit 6 N 6N
128-QAM Septabit 7 N 7N
256-QAM Octabit 8 N 8N
20
Sampling Pulse Amplitude Modulation (PAM)
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Sampling Pulse Amplitude Modulation (PAM)
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Quantized PAM Signal
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Figure 3.11Illustration of the quantization
process. (Adapted from Bennett, 1948, with
permission of ATT.)
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Figure 5-20-continued
From Analog to PCM
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Figure 5-20-continued
From Analog to PCM
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Figure 5-20-continued
From Analog to PCM
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Figure 5-19
Pulse Coded Modulation
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Nyquists Sampling Theorem
A band-limited signal of finite energy, which has
no frequency components higher than W Hertz, may
be completely described by specifying the values
of the signal at instants of time separated by
(1/2W) seconds or can be recovered from a
knowledge of its samples taken at a rate of 2W
samples per second.
fs 2 W
Sampling frequency
Bandwidth of signal
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Impact of Sampling on the Frequency Domain
30
Impact of Sampling on the Frequency Domain
Message signal Frequency Content
Frequency Content of the sampled message signal
fs 2 W
31

• Revisit Analog Modulation Schemes
• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• How to produce AM Signal?

32
Amplitude Modulation
33
Amplitude Modulation
Modulating signal
VAM(t)
vS(t)
cos ?C t
vC
Carrier Amplitude
Carrier Frequency
Carrier Signal
Message Signal or modulating signal
Modulated Signal
Modulation Index
34
Amplitude Modulation

• Modulation Index M is determined by the peak
  amplitudes of the carrier and the modulating
  signal.
• In practice, carrier signal amplitude vC is
  usually fixed and the M ratio is changed by
  varying the amplitude of the modulating signal
  vS.
• Hence, higher vS produce higher M but M lt 1.
• M is kept as high as possible to ensure good SNR
  of the received AM signal for recovery.
• When M gt 1, over-modulated carrier signal
  distorts the information Clipping or
  saturation.

35
Amplitude Modulation
Envelope of the modulated signal has the same
shape with the message signal.
Envelope is distorted
Illustrating the amplitude modulation process.
(a) Baseband signal vs(t). (b) AM wave for M lt
1 for all t. (c) AM wave for M gt 1 for some t.
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AM Double-sided band (DSB)
Action To suppress the carrier
37
Amplitude Modulation

• AM is the earliest type of modulation in history.
• Its main advantage is its simplicity. linear
  modulation technique
• AM is wasteful in power consumption. Although the
  carrier signal does not carry any information, it
  is still transmitted.
• AM is wasteful in bandwidth usage. The upper
  sideband is reflection of the lower sideband. One
  sideband is sufficient to express the frequency
  content of the message signal. Yet, AM still
  transmits one unnecessary sideband.

38
Spectrum of AM wave
(a) Spectrum of AM Signal both carrier and
double-sided bands
v
f
fc-fS
fS
fc
fcfS
(b) Spectrum of Double-Sided Band - Carrier
Suppression (DSB-SC)
v
f
fc-fS
fc
fcfS
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Double Sideband-Suppressed Carrier Modulation
(DSB-SC)
Modulating signal
DSB-SC signal
vS(t)
cos ?C t
Carrier Frequency

• Balance Modulator

vS(t)
DSB-SC
Carrier Oscillator
-90o
vC(t)
-90o
40
Double Sideband-Suppressed Carrier Modulation
(DSB-SC)
modulating signal
DSB-SC signal
41
Single Sideband-Suppressed Carrier Modulation
(SSB-SC)
Sideband filter (crystal filter)
Modulating signal
DSB-SC signal
SSB-SC signal
vS(t)
cos ?C t
Carrier Frequency
Bandpass filter applied at the DSB-SC signal to
generate SSB-SC signal.
Problem It is very difficult and costly to
design a bandpass filter that is sharp enough to
select only one sideband !
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Demodulation of AM signal
Modulating signal
DSB-SC signal
cos ?C t
Carrier Frequency

• Balance Modulator

vS(t)
DSB-SC
Carrier Oscillator
-90o
vC(t)
-90o
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Demodulation of AM signal

• Most basic Envelope detector (for AM signal
  only)

Charging/Discharging voltage
AM signal
Cc

vAM(t)

vs(t)
diode D
To remove DC component smoothen vs(t)
C
R

• As VAM(t) increases in amplitude, the diode
  conducts (forward bias) and capacitor C start to
  charge-up very quickly to 1st peak vp1 with a
  time constant ? Cr, where r is the diodes
  forward resistance (usually very small when diode
  is conducting).
• As VAM(t) decreases in amplitudes, the diode
  switch-off (reverse bias) and capacitor C start
  to discharge slowly with a time constant ? CR,
  where R must be greater than r.
• When VAM(t) increases again, D conducts and C
  charges up rapidly to 2nd peak vp2 and when
  VAM(t) decreases again D is off and C discharges
  slowly and this is repeated according to the
  amplitude of VAM(t) signal.
• If CR is too small, C discharge too rapidly
  results in ripple amplitude in demodulated
  output.
• If CR is too large, C discharge too slowly vs(t)
  fails to follow the envelope results in
  distortion (or diagonal clippling) in demodulated
  output.
• Hence, time constant ? must be optimum.

44
Optimum AM Demodulation
Ripple amplitude in AM Demodulation RC too small
Diagonal Clipping/distortion in AM Demodulation
RC too large
45
Demodulation of (DSB-SC) signal (1)

• Synchronous detection

Low Pass Filter
Recovered modulating signal
DSB-SC signal
cos ?C t
local oscillator

• Local oscillator produce the exactly coherent
  oscillation output that is synchronized with the
  original carrier in both frequency and phase.
• The output is then filter by low-pass filter that
  only allowed the desired signal to pass through.

Desired signal
Unwanted signal
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Demodulation of (DSB-SC) signal (2)

• Costal Loop / Phase Lock Loop (PLL)

Output
LPF
DSB-SC
Loop filter
VCO
-90o
-90o
LPF

• The frequency fc is know a priori to the
  demodulator and generated by the voltage control
  oscillator, VCO.
• PLL circuit (VCO Loop filter) try to lock the
  phase so that local oscillation is synchronized
  with original fc.of the DSBSC signal.
• Once synchronization is achieved, the difference
  in phase will be eliminated, thereby,
  recover the modulating signal.

47
Frequency Modulation (FM)
vs
fC
fi
In FM, the information is conveyed by varying the
frequency of the carrier signal fC in step with
the instantaneous amplitude of the modulating
signal vs.
48
Frequency Modulation (FM)

• FM signal is produced by a frequency modulator
  which converts the voltage variation in the
  modulating signal vs to a frequency variation of
  the carrier signal
• The instantaneous frequency fi is the sum of
  carrier frequency fC and the frequency
  deviation as the result of the
  amplitude-frequency conversion.

fi
vs(t)
Frequency Modulator

• When no modulating signal is applied, the output
  frequency is the same as the carrier frequency
  since no deviation is observed.
• When a modulating signal is applied, the
  instantaneous output frequency fi will start to
  vary/deviate from fc with the amount of .
• The conversion can be seen from the graph

fi
kf
fc
Conversion gain
vs
0
49
Frequency Modulation (FM)
50
Frequency Modulation (FM)
51
Frequency Modulation (FM)

• Therefore, FM signal can be expressed as

Carrier frequency is varied or deviated by the
amount of
FM is a non-linear modulation. FM signal
envelope is constant.

• ? controls the amount of frequency change in FM
  signal.
• In FM, ? can be greater than 1 (? gt 1), since
  can be set independent of fs and both values
  are not bounded by fC.
• However, fs must be kept smaller than fC in
  order for FM to work successfully.

52
Frequency Modulation
Carsons Rule
The transmission bandwidth required by a
frequency modulated signal is given below.
FM modulation index
Maximum frequency deviation
53
Example
A message signal with a bandwidth of 15 KHz is to
be used to frequency modulate a carrier signal at
400 KHz. Given that maximum frequency deviation
is 75 KHz. According to Carsons Rule, what is
the transmission bandwidth required for the
frequency modulated signal?
1) message bandwidth ?
2) maximum frequency deviation ?
3) modulation index ?
4) Using Carsons rule, the required transmission
bandwidth
54
Tutorial
1- What is Amplitude Modulation ?
Amplitude modulation is the process by which the
amplitude of a carrier signal is varied according
to a message signal.
2- What frequency range will be covered by a 412
KHz carrier signal after it has been amplitude
modulated by an audio signal that is bandlimited
to 24 KHz ?
Due to amplitude modulation, the frequency
spectrum of the audio signal will shift to the
carrier signal frequency. The frequency range
from (412-24) KHz to (41224) KHz will be covered
by the amplitude modulated signal.
55
Tutorial
3- Consider the video signal that has a frequency
content between 0 Hz and 6 MHz. What is the
required transmission bandwidth if Frequency
Modulation is used with a maximum frequency
deviation of 30 MHz according to Carsons Rule ?
maximum frequency deviation ?
modulation index ?
56
Tutorial
With aid of diagram, explain the process of
amplitude modulation? Your answer should include
the carrier signal, modulating signal and the AM
signal itself.
57
Tutorial
4- What is Amplitude Shift Keying (ASK)?
ASK is a digital modulation technique where the
amplitude of a carrier signal is varied to
transmit ones and zeros.
5- What is Phase Shift Keying (PSK)?
PSK is a digital modulation technique where the
phase of a carrier signal is varied to transmit
ones and zeros.
6- What is Frequency Shift Keying (FSK)?
FSK is a digital modulation technique where the
frequency of a carrier signal is varied to
transmit ones and zeros.
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Tutorial
7- Sketch the ASK modulated signal for a bit
pattern of 01100101. Use a cosine wave as the
carrier signal!
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Tutorial
8- Sketch the PSK modulated signal for a bit
pattern of 01100101. Use a cosine wave as the
carrier signal!
60
Tutorial
Briefly explain the Pulse Amplitude Modulation
(PAM) and Quantisation process.
PAM converts the analog signal to a series of
pulse-trains with different amplitude
corresponding to the amplitude of the analog
signal at different interval in time.
Quantisation is a process to convert these
pulse-trains amplitude from analog value to
discrete value by binary level representation.
The number levels (L) that can be represented is
corresponding to the number of bit (N) used. L
2N
61
Tutorial
With the aid of block diagram, describe how a
analog signal is sent using a digital system with
PAM.