Transcript AMPLITUDE MODULATION

Signal Encoding Techniques
(modulation and encoding)
Analog
data to analog signal
(AM, FM, PM)
Digital
data to analog signal
(ASK, FSK, BPSK, QAM)
Analog
data to digital signal
(PCM, DM)
Digital
data to digital signal
(line codes)
DCTC, By Ya Bao
1
Analog Signals
DCTC, By Ya Bao
2
Digital Signals
DCTC,
By Ya
3
Analog and Digital Transmission
AMPLITUDE MODULATION
DCTC, By Ya Bao
5

Modulation


The process by which some characteristics of a carrier
wave is varied in accordance with an informationbearing signal.
Continuous-wave modulation
Amplitude modulation
 Frequency modulation


AM modulation family
Amplitude modulation (AM)
 Double sideband-suppressed carrier (DSB-SC)
 Single sideband (SSB)
 Vestigial sideband (VSB)

DCTC, By Ya Bao
6
AMPLITUDE MODULATION
Carrier wave: is a waveform (usually sinusoidal) that is modulated (modified)
with an input signal for the purpose of conveying information. This carrier wave
is usually a much higher frequency than the input signal.
1.

DEFINING AM
A carrier wave whose amplitude is varied in
proportion to the instantaneous amplitude of a
modulating voltage
GENERATING THE AM
nonlinear device: diode or transistor biased in its
nonlinear region
2.
DCTC, By Ya Bao
7
DCTC, By Ya Bao
8
3. ANALYSIS OF THE AM WAVE
vc  Vc sin 2f c t
m
m
v  Vc sin 2f c t  Vc cos 2 ( f c  f s )t  Vc cos 2 ( f c  f s )t
2
2
DCTC, By Ya Bao
9
DCTC, By Ya Bao
10
4. Different Carriers and AM
Carriers are spaced at 20 kHz, beginning at 100kHz.
Each carrier is modulated by a signal with 5kHz
bandwidth. Is there interference from sideband overlap?
DCTC, By Ya Bao
11
5. MODULATION INDEX AND SIGNAL POWER
Vm
Vmax  Vmin
m

Vc
Vmax  Vmin
DCTC, By Ya Bao
12
Moduiation Index and Power
2
carr
2
V
2
c
(Vc / 2 )
V
Pc 


R
R
2R
PLSB  PUSB
2
Vc
m


2R
4
2
C
2
2
2
PT
m  2(  1)
PC
V
m
m
PT 
(1 
)  Pc (1 
)
2R
2
2
PT
m
 1
Pc
2
DCTC, By Ya Bao
13
Current Calculations
2
IT
m
 1
Ic
2
Example
A carrier of 1000 W is modulated with a resulting
modulation index of 0. 8. What is the total power?
What is the carrier power if the total power is 1000 W
and the modulation index is 0.95?
DCTC, By Ya Bao
14
6.2 Double Sideband Suppressed Carrier
(DSBSC)
When the carrier is reduced, this is called doublesideband suppressed-carrier AM, or DSB-SC. If the
carrier could somehow be removed or reduced, the
transmitted signal would consist of two informationbearing sidebands, and the total transmitted power
would be information
DCTC, By Ya Bao
15
6.3 Single-Sideband (SSB)

suppressing the carrier and one of the sidebands
DCTC, By Ya Bao
16
DCTC, By Ya Bao
17
6.4 Filtering the SSB LSB or USB

Dual Conversion: up-converting the modulating
frequency twice and selecting the upper or lower
sideband for transmission.
DCTC, By Ya Bao
18
AM: Features and Drawbacks:
the AM signal is greatly affected by noise
impossible to determine absolutely the original
signal level
conventional AM is not efficient in the use of
transmitter power
AM is useful where a simple, low-cost
receiver and detector is desired
DCTC, By Ya Bao
19
Angle Modulation
DCTC, By Ya Bao
20
ANGLE MODULATION:
The intelligence of the modulating signal can be
conveyed by varying the frequency or phase of the
carrier signal. When this is the case, we have
angle modulation, which can be subdivided into
two categories: frequency modulation (FM), and
phase modulation (PM).
DCTC, By Ya Bao
21
Frequency Modulation. The carrier's
instantaneous frequency deviation from its
unmodulated value varies in proportion to the
instantaneous amplitude of the modulating signal.
eFM  Ac sin(ct  m f sin mt )
Phase Modulation. The carrier's instantaneous
phase deviation from its unmodulated value
varies as a function of the instantaneous
amplitude of the modulating signal;
ePM  Ac sin(ct  m sin mt )
DCTC, By Ya Bao
22
FIGURE 4-1 The FM and PM waveforms for sine-wave modulation: (a) carrier
wave; (b) modulation wave; (c) FM wave; (d) PM wave. (Note: The derivative of
the modulating sine wave is the cosine wave shown by the dotted lines. The
PM wave appears to be frequency modulated by the cosine wave.)
DCTC, By Ya Bao
23
MODULATION INDEX

modulation index for an FM signal
mf 

fm
δ = maximum frequency deviation of the carrier caused
by the amplitude of the modulating signal
fm = frequency of the modulating signal
DCTC, By Ya Bao
24
FREQUENCY ANALYSIS OF THE FM WAVE
eFM  Ac J 0 sin  ct
 Ac {J 1 (m f )[sin(c  m )t  sin(c  m )t ]}
 Ac {J 2 (m f )[sin(c  2m )t  sin(c  2m )t ]}
 Ac {J 3 (m f )[sin(c  3m )t  sin(c  3 m )t ]}
 ..., etc

where: eFm = the instantaneous amplitude of the
modulated FM wave
Ac = the peak amplitude of the carrier
Jn = solution to the nth order Bessel function for a
modulation index mf.
mf = FM modulation index, Δf/fm
DCTC, By Ya Bao
25
DCTC, By Ya Bao
26
Spectral components of a carrier of frequency, fc, frequency modulated by
a sine wave with frequency fm
DCTC, By Ya Bao
27
FM signal characters
• The FM wave is comprised of an infinite number of
sideband components
• bandwidth of an FM signal must be wider than that of
an AM signal
• As the modulation index increases from mf = 0, the
spectral energy shifts from the carrier frequency to
an increasing number of significant sidebands.
• Jn(mf) coefficients, decrease in value with increasing
order, n.
• negative Jn(mf) coefficients imply a 1800 phase
inversion.
DCTC, By Ya Bao
28
Carrier Frequency Eigenvalues

in some cases the carrier frequency component, JO, and
the various sidebands, Jn go to zero amplitudes at specific
values of m. These values are called eigenvalues.
DCTC, By Ya Bao
29
Bandwidth Requirements for FM

The higher the modulation index, the greater the
required system bandwidth
BW  2(n  f m )
where n is the highest number of significant
(least 1%, or -40 dB; (20 log 1/100 ), of the voltage of the unmodulated carrier)
sideband components and fm is the highest
modulation frequency.
Carson's Rule
BW  2(  f m )  2 f m (1  m f )
DCTC, By Ya Bao
30
Amplitude versus frequency spectrum for various modulation indices (fm
fixed, & varying): (a) mf = 0.25; (b) mf = 1; (c) mf = 2; (d) mf = 5; (e) mf = 10.
DCTC, By Ya Bao
31
Warren Hioki
Telecommunications,
Fourth Edition
DCTC, By Ya Bao
32
Copyright ©2001 by Prentice-Hall, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 4-6
Commercial FM broadcast band.
DCTC, By Ya Bao
33
Commercial FM broadcast band
•
The maximum permissible carrier deviation, δ, is ±75
kHz
•
Modulating frequencies (voice or music) is ranging from
50 Hz to 15 kHz
•
The modulation index can range from as low as 5 for fm
= 15 kHz (75 kHz/15 kHz) to as high as 1500 for fm = 50
Hz (75 kHz/50 Hz).
•
The ±75-kHz carrier deviation results in an FM
bandwidth requirement of 150 kHz for the receiver.
•
A 25-kHz guard band above and below the upper and
lower FM sidebands.
•
Total bandwidth of one channel is 200Hz.
DCTC, By Ya Bao
34
Narrowband FM (NBFM)




NBFM uses low modulation index values, with a
much smaller range of modulation index across all
values of the modulating signal.
An NBFM system restricts the modulating signal to
the minimum acceptable value, which is 300 Hz to
3 KHz for intelligible voice.
10 to 15 kHz of spectrum.
Used in police, fire, and Taxi radios, GSM, amateur
radio, etc.
DCTC, By Ya Bao
35
POWER IN THE FM WAVE


power of the unmodulated carrier
Vcrms
PT 
R
For a modulated carrier
2
PT  PJ 0  PJ1  PJ 2  PJ 3  ...  PJ n

2
J0
V
R
DCTC, By Ya Bao

2
J1
2V
R

2
J2
2V
R

2
J3
2V
R
 ... 
2
Jn
2V
R
36
FM NOISE
 Increased
bandwidth of an FM – to enhance the signalto-noise ratio (SNR). Advantages of FM over AM.
 To
take this advantage, large mf is necessary– high
order sidebands are important – wider bandwidth is
required.
 Phase
Analysis of FM Noise
VN
  sin
Vc
1
where α = the maximum phase deviation of the carrier frequency caused by the
noise
VN = noise voltage
Vc= carrier voltage
DCTC, By Ya Bao
37
Phasor addition of noise on an FM signal’s carrier frequency causes a phase
shift, whose maximum value is .
Warren Hioki
Telecommunications,
Fourth Edition
DCTC, By Ya Bao
38
Copyright ©2001 by Prentice-Hall, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The ratio of carrier voltage to noise voltage,
Vc
Vn
is the SNR (voltage)
Vc
SNR 
VN
1
  sin
SNR
1
α represents the equivalent modulation index
produced by the noise.
 N    fm
SNRFM
DCTC, By Ya Bao


N
39
• The effect of noise on an FM carrier signal is
directly proportional to the modulation frequency fm.
• Increasing fm, degrades the


SNR 

 N   fm
Voice, data, and music contain many frequencies,
which are distributed throughout the given
modulation passband. Therefore, the SNR is not
uniform throughout.
To maintain a flat SNR, some techniques are
employed.
DCTC, By Ya Bao
40