Analog data to analog signal (AM, FM, PM) Digital data to analog signal (ASK, FSK, BPSK, QAM) ...
Transcript of Analog data to analog signal (AM, FM, PM) Digital data to analog signal (ASK, FSK, BPSK, QAM) ...
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)
Signal Encoding Techniques (modulation and encoding)
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Analog Signals
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Digital Signals
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Analog and Digital Transmission
AMPLITUDE MODULATION
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Modulation The process by which some characteristics of a
carrier wave is varied in accordance with an information-bearing 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)
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AMPLITUDE MODULATION
1. DEFINING AM A carrier wave whose amplitude is varied
in proportion to the instantaneous amplitude of a modulating voltage
2. GENERATING THE AM nonlinear device: diode or transistor biased in
its nonlinear region
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.
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3. ANALYSIS OF THE AM WAVE tfVv ccc 2sin
tffVm
tffVm
tfVv sccscccc )(2cos2
)(2cos2
2sin
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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?
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5. MODULATION INDEX AND SIGNAL POWER
minmax
minmax
VV
VV
V
Vm
c
m
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Moduiation Index and Power
R
V
R
V
R
VP cccarrc 2
)2/( 222
42
2m
R
VPP cUSBLSB
)2
1()2
1(2
222 mP
m
R
VP c
CT
21
P
P 2
c
T m )1(2
C
T
P
Pm
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Current Calculations
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2m
I
I
c
T
ExampleA 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?
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6.2 Double Sideband Suppressed Carrier (DSBSC)
When the carrier is reduced, this is called double-sideband suppressed-carrier AM, or DSB-SC. If the carrier could somehow be removed or reduced, the transmitted signal would consist of two information-bearing sidebands, and the total transmitted power would be information
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6.3 Single-Sideband (SSB) suppressing the carrier and one of the sidebands
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6.4 Filtering the SSB LSB or USB Dual Conversion: up-converting the mod ulating
frequency twice and selecting the upper or lower sideband for transmission.
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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
Angle Modulation
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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).
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Frequency Modulation. The carrier's instantaneous frequency deviation from its unmodulated value varies in proportion to the instantaneous amplitude of the modulating signal.
Phase Modulation. The carrier's instantaneous phase deviation from its unmodulated value varies as a function of the instantaneous amplitude of the modulating signal;
)sinsin( tmtAe mfccFM
)sinsin( ttAe mmccPM
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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.)
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MODULATION INDEX
modulation index for an FM signal
mf f
m
δ = maximum frequency deviation of the carrier caused by the amplitude of the modulating signal
fm = frequency of the modulating signal
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FREQUENCY ANALYSIS OF THE FM WAVE
where: eFm = the instantaneous amplitude of the modulated FM waveAc = the peak amplitude of the carrierJn = solution to the nth order Bessel function for a modulation index mf.
mf = FM modulation index, Δf/fm
etc ...,
]})3sin()3)[sin(({
]})2sin()2)[sin(({
]})sin())[sin(({
sin
3
2
1
0
ttmJA
ttmJA
ttmJA
tJAe
mcmcfc
mcmcfc
mcmcfc
ccFM
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Spectral components of a carrier of frequency, fc, frequency modulated by a sine wave with frequency fm
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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.
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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.
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Bandwidth Requirements for FM The higher the modulation index, the greater
the required system bandwidth
)(2 mfnBW 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
)1(2)(2 fmm mffBW
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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.
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FIGURE 4-6 Commercial FM broadcast band.
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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.
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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.
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POWER IN THE FM WAVE
power of the unmodulated carrier
For a modulated carrierR
VP crmsT
2
R
V
R
V
R
V
R
V
R
V
PPPPPP
n
n
JJJJJ
JJJJJT
22222 2...
222
...
3210
3210
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FM NOISE
Increased bandwidth of an FM – to enhance the signal-to-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
c
N
V
V1sin
where α = the maximum phase deviation of the carrier frequency caused by the noise
VN = noise voltage
Vc= carrier voltage
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Phasor addition of noise on an FM signal’s carrier frequency causes a phase shift, whose maximum value is .
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Copyright ©2001 by Prentice-Hall, Inc.Upper Saddle River, New Jersey 07458
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The ratio of carrier voltage to noise voltage, is the SNR (voltage)n
c
V
V
SNR
V
VSNR
N
c
1sin 1
α represents the equivalent modulation index produced by the noise.
mN f
NFMSNR
40
mN fSNR
• The effect of noise on an FM carrier signal is directly proportional to the modulation frequency fm.
• Increasing fm, degrades the
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.
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