Post on 18-Dec-2015
transcript
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Modulations/demodulations in Transmitters/Receivers
Amplitude modulation (AM)
Angle modulation – Frequency, phase
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AM modulation
AM has the advantage of being usable with very simple modulators and demodulators
Disadvantages include poor performance in the presence of noise and inefficient use of transmitter power
Applications: broadcasting, aircraft communications in the VHF frequency range
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Full carrier AM
V(t) = (Ec + em)sin(ωc x t)
Example 3.1
Modulation index m = Em / Ec
Over modulation
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Optical Carrier fc
Modulating signal em
DC bias
AM modulation circuits
RF modulation
Optical modulation
Modulating signal em
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Modulation index for multiple modulating frequencies
mT = sqrt (m12 + m2
2 + …)
Example 3.3
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Measurement of modulation index
m = (Emax – Emin) / (Emax + Emin)
Example 3.4
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Full carrier AM: frequency domain
V(t) = Ec sin(ωc t) carrier
+ mEc/2 cos(ωc - ωm )t left sideband
– m Ec/2 cos(ωc + ωm )t right sideband
Example 3.5
Ec
m/2Ec
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Bandwidth and power relationships
Bandwidth:
B = 2 fm
Power relationship:
Plsb = m2 /4 Pc
Pt = Pc (1 + m2/2)
Ec
m/2Ec
2fm
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Some observations
The total power in an AM signal increases with modulation, reaching a value 50% greater than that of the un-modulated carrier for 100% modulation
The extra power with modulation goes into the sidebands: the carrier power does not change with modulation
The useful power is rather small, reaching a maximum of 1/3 of the total signal power . For this reason, AM transmission is more efficient when the modulation index is as close to 1
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Measuring the modulation index in the frequency domain
M = 2 x sqrt(Plsb / Pc)
Example 3.11
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Quadrature AM
AM modulator
AM modulator
Phase shifter
Cos
Sin
Demodulation is the reverse process
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QAM demodulation
Carrier recovery
Phase shifter
Cos
Sin
To study the case when there exists phase shift from the carrier
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Suppressed-Carrier AM
In normal AM, two-third of the transmitted power is found in the carrier
Suppressed-carrier AM removes the carrier
Psb = 0.5 Pc = 1/3 Pt
Pc
1/6 Pt
Pt/21/6 Pt
Pt/2
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0
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0 100 200 300
Time (ps)
Am
plit
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e (a
.u.)
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0 100 200 300
Time (ps)
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plit
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.u.)
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1551 1552 1553 1554 1555
Wavelength (nm)
Po
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(d
B)
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1551 1552 1553 1554 1555
Wavelength (nm)
Po
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r (d
B)
CSRZ RZ PW = 11.4 ps, ER = 13.7 dB PW = 9.2 ps, ER = 18.0 dB
Streak camera trace
Optical spectra
Practical examples: RZ and CSRZ
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Single-sideband AM
Two sidebands of an AM signal are mirror images
Removing one sideband reduces the bandwidth, and improves the signal-to-noise ratio
Pt/2 Pt/2 DSBSC SSB
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Power in suppressed-carrier signals
Peak envelop power is used for suppressed-carrier signals:
PEP = [Vp / sqrt(2)]2 / RL
Example 3.11
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Matlab simulation
P128, example of AM modulation
A simple AM modulator
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Angle modulation
Angle modulation can be divided into frequency (FM) and phase modulation (PM)
Both FM and PM are widely used in communication systems
The most important advantage of FM or PM over AM is the possibility of improved signal to noise ratio
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Frequency modulation
Frequency
Implementation
Amp VCO
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Frequency modulation
Cos(ωc t + θ), ωc is the modulating signal
Frequency deviation:
fsig = fc + kf Em(t)
Where kf is the modulator deviation constant
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Example 4.1
kf = 30 kHz/v, carrier frequency is 175 MHz, find out the frequency for an modulating signal equal to: 150 mV and –2V
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Frequency modulation index
Peak frequency deviation δ = kf Em
Fsig( fc + δ sin ωm t)
Frequency modulation index mf = δ / fm
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Example 4.3
An FM transmitter operates at its maximum deviation of 75 kHz, find out the modulation index for a sine modulation signal with a frequency of 15 kHz and 50 Hz.
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Phase modulation
kp = Φ/em
Kp: phase modulator sensitivity
Φ: phase deviation
em: signal amplitude
θ(t) = θc + kp em(t)
in case of sin signal: θ(t) = θc + mp sinωmt
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Relationship between frequency modulation and phase modulation
f
Phase shift
θ = ωt = Integral(ω dt)
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Implementation of a phase modulator
Amp VCO
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