Introduction to Communication Prepared By Dr. Hany Taher Modified by: Dr. Mouaaz Nahas.
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Transcript of Introduction to Communication Prepared By Dr. Hany Taher Modified by: Dr. Mouaaz Nahas.
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Introduction to Communication
Prepared By Dr. Hany TaherPrepared By Dr. Hany Taher
Modified by: Dr. Mouaaz NahasModified by: Dr. Mouaaz Nahas
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Text Book
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ContentsContents
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Contents (Cont.)Contents (Cont.)
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Contents (Cont.)Contents (Cont.)
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Comm. System (Cont.)Comm. System (Cont.)
)
that converted by input transducer
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Comm. System (Cont.)Comm. System (Cont.)
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Comm. System (Cont.)Comm. System (Cont.)
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Can be eliminatedCan be eliminated
Comm. System (Cont.)Comm. System (Cont.)
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Cannot be eliminatedCannot be eliminated
Comm. System (Cont.)Comm. System (Cont.)
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Comm. System (Cont.)Comm. System (Cont.)
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Analog & Digital MessagesAnalog & Digital Messages
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Analog & Digital Messages (Cont.)Analog & Digital Messages (Cont.)
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Noise immunity of Digital SignalsNoise immunity of Digital Signals
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Distorted signalDistorted signal
Distorted noisy signalDistorted noisy signal
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Due to channelDue to channel
Noise immunity of Digital Signals (Cont.)Noise immunity of Digital Signals (Cont.)
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Noise immunity of Digital Signals (Cont.)Noise immunity of Digital Signals (Cont.)
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Viability of Regenerative Repeaters in Viability of Regenerative Repeaters in Digital CommunicationDigital Communication
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Viability of Regenerative Repeaters in Viability of Regenerative Repeaters in Digital Communication (Cont.)Digital Communication (Cont.)
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Signal to noise ratio
Signal Bandwidth & SNRSignal Bandwidth & SNR
A measure of the width of a range of frequencies, measured in hertz (Hz).
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Sinusoidal WaveformSinusoidal Waveform
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ModulationModulation
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DemodulationDemodulation
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AM (Cont.)AM (Cont.)
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AM (Cont.)AM (Cont.).
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LetLet A=1A=1
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AM (Cont.)AM (Cont.)
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AM (Cont.)AM (Cont.).
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AM DemodulationAM Demodulation
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AM Demodulation (Cont.)AM Demodulation (Cont.)
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Example on AMExample on AM
Solution
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Example on AM (Cont.)Example on AM (Cont.)
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Example on AM (Cont.)Example on AM (Cont.)
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Modulators (Cont.)Modulators (Cont.)
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Modulators (Cont.)Modulators (Cont.).
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Modulators (Cont.)Modulators (Cont.).
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Modulators (Cont.)Modulators (Cont.).
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Modulators (Cont.)Modulators (Cont.)
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Modulators (Cont.)Modulators (Cont.)
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Modulators (Cont.)Modulators (Cont.).
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Modulators (Cont.)Modulators (Cont.)
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Modulators (Cont.)Modulators (Cont.).
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Modulators (Cont.)Modulators (Cont.)
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Modulators (Cont.)Modulators (Cont.)
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Modulators (Cont.)Modulators (Cont.)
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Modulators (Cont.)Modulators (Cont.)
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DSB-SC Demodulation (Cont.)DSB-SC Demodulation (Cont.).
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SolutionSolution
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Example (Cont.)Example (Cont.)
LPFLPF
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SolutionSolution
Example: Frequency MixerExample: Frequency Mixer
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Example: Frequency Mixer (Cont.)Example: Frequency Mixer (Cont.)
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ExercisesExercises4.2.14.2.1
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Exercises (Cont.)Exercises (Cont.)SolutionSolution
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Exercises (Cont.)Exercises (Cont.)
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Exercises (Cont.)Exercises (Cont.)
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Exercises (Cont.)Exercises (Cont.)
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Exercises (Cont.)Exercises (Cont.)
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Exercises (Cont.)Exercises (Cont.)
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Exercises (Cont.)Exercises (Cont.)4.2-44.2-4
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Exercises (Cont.)Exercises (Cont.)
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Exercises (Cont.)Exercises (Cont.)
Wanted Unwanted
SolutionSolution
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Exercises (Cont.)Exercises (Cont.)
Band pass Filter at wc
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Exercises (Cont.)Exercises (Cont.)
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Exercises (Cont.)Exercises (Cont.)
SolutionSolution
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Exercises (Cont.)Exercises (Cont.)
kk
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Transmitter may be very far, i.e, thousands of KmsTransmitter may be very far, i.e, thousands of Kms
Expensive
. Solution is to use Amplitude modulation:
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AM (Cont.)AM (Cont.). DisadvantageDisadvantage
..
More power is neededMore power is needed
..
1. Point to point communications :1. Point to point communications :
• One transmitter for every receiver. One transmitter for every receiver. • Complexity in the receiver is justified. Complexity in the receiver is justified.
2. Broadcast communications: 2. Broadcast communications:
• Single transmitter and multi receivers. Single transmitter and multi receivers. • Better economically to have one expensive high-power Better economically to have one expensive high-power transmitter and simpler less expensive multiple transmitter and simpler less expensive multiple receivers.receivers. 73
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AM (Cont.)AM (Cont.).
.
. Remember
Compare
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AM (Cont.)AM (Cont.)
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AM (Cont.)AM (Cont.)
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AM (Cont.)AM (Cont.)Two conditions for the envelop detection of an Two conditions for the envelop detection of an AM signal are:AM signal are:1)1) ffcc > bandwidth of> bandwidth of m(t) m(t)
2)2)A A ++ m(t) m(t) ≥≥ 0 0 for all for all tt
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If If A + m(t) A + m(t) is not positive for all is not positive for all t, m(t) t, m(t) cannot be cannot be recovered from the enveloprecovered from the envelop \ A + m(t)\. \ A + m(t)\.
If If m(t) m(t) ≥≥ 0 0 for all for all t, A=0 t, A=0 already satisfies the already satisfies the condition condition A A ++ m(t) m(t) ≥≥ 0. 0. No need to add any carrier because the envelop of No need to add any carrier because the envelop of the DSB-SC signal the DSB-SC signal m(t) m(t) coscosωωcctt is is m(t).m(t). This signal can This signal can
be detected by envelop detection.be detected by envelop detection.
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AM (Cont.)AM (Cont.)Let, mLet, mpp the peak amplitude the peak amplitude
The modulation indexThe modulation index
A< mA< mpp µ > 1 (overmodulation)µ > 1 (overmodulation)78
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SolutionSolution
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Example (Cont.)Example (Cont.)
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Sidebands and AM efficienciesSidebands and AM efficiencies
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Time mean square value
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Sidebands and AM efficiencies (Cont.)Sidebands and AM efficiencies (Cont.)
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SolutionSolution
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Sidebands and AM efficiencies (Cont.)Sidebands and AM efficiencies (Cont.)
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Generation of AM Signals (Cont.)Generation of AM Signals (Cont.)
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Rectifier Detector (Cont.)Rectifier Detector (Cont.)
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Rectifier Detector (Cont.)Rectifier Detector (Cont.)
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Rectifier Detector (Cont.)Rectifier Detector (Cont.)
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Rectifier Detector (Cont.)Rectifier Detector (Cont.)
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Envelope DetectorEnvelope Detector
1. First AM positive cycle1. First AM positive cycle
Capacitor is charged until the peak valueCapacitor is charged until the peak value
2. The AM signal falls below the peak2. The AM signal falls below the peak
Diode OnDiode On
Diode is OffDiode is Off
Capacitor is discharged into the resistance with time Capacitor is discharged into the resistance with time constant ‘RC’ and its voltage starts to decrease from the constant ‘RC’ and its voltage starts to decrease from the peak valuepeak value 91
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3. The AM signal now is in its negative cycle3. The AM signal now is in its negative cycle
Diode is still offDiode is still off
Capacitor is Capacitor is still still discharged into the resistance discharged into the resistance and its voltage and its voltage continuescontinues in decreasing from the peak value in decreasing from the peak value
4. The AM signal now is going into 24. The AM signal now is going into 2ndnd positive cycle positive cycle
Diode is still off until the value of AM signal becomes Diode is still off until the value of AM signal becomes larger than voltage on the capacitor, at this moment,larger than voltage on the capacitor, at this moment,Diode is onDiode is on
Capacitor is charged until the peak value, and so Capacitor is charged until the peak value, and so on. The above steps are repeated againon. The above steps are repeated again 92
Envelope Detector (Cont.)Envelope Detector (Cont.)
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RipplesRipples
To decrease the ripples, use high ‘RC’To decrease the ripples, use high ‘RC’ Take CareTake CareVery high RC does not accurately follow the Very high RC does not accurately follow the envelope envelope
TTCC << RC << T << RC << Tm m (1/B)(1/B)
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Envelope Detector (Cont.)Envelope Detector (Cont.)
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• In DSB (SC and AM), each USB and LSB contains complete information about m(t).
• So DSB modulation requires twice the bandwidth of baseband to transmit (spectral redundancy).
• To improve the spectral efficiency, we have two schemes:– Single-Sideband (SSB).– Quadrature Amplitude Modulation (QAM).
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Bandwidth-Efficient AMBandwidth-Efficient AM
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As mentioned earlier, DSB occupies twice of As mentioned earlier, DSB occupies twice of bandwidth of baseband bandwidth of baseband (this is a disadvantage).(this is a disadvantage).
Solution Solution Two baseband signal on the same carrier frequency, Two baseband signal on the same carrier frequency, how??!!!!!how??!!!!!
There is There is ππ/2 (radian) phase shift between the two /2 (radian) phase shift between the two carriers, i.e. Cosine and Sine as example.carriers, i.e. Cosine and Sine as example.
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QAM (or Quadrature Multiplexing) transmits two DSB QAM (or Quadrature Multiplexing) transmits two DSB signals using two carriers of the same frequency but in signals using two carriers of the same frequency but in phase quadrature.phase quadrature.
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QAM (Cont.)QAM (Cont.)
Two baseband signalsTwo baseband signals
Carrier signalCarrier signal
Phase Shifted Phase Shifted Carrier signalCarrier signal
Modulated SignalModulated Signal
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Phase Phase ShifterShifter
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QAM (Cont.)QAM (Cont.)
In-phase (I)In-phase (I)channelchannel
Quadrature (Q)Quadrature (Q)channelchannel
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QAM (Cont.)QAM (Cont.)
The last two terms in xThe last two terms in x11(t) form a QAM signal with 2(t) form a QAM signal with 2ωωcc as as
the carrier frequency. They are suppressed by the low-pass the carrier frequency. They are suppressed by the low-pass filter yielding mfilter yielding m11(t).(t).
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Obtain an expression for xObtain an expression for x22(t)????(t)????
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QAM (Cont.)QAM (Cont.)Note that QAM must be totally synchronous.Note that QAM must be totally synchronous.The problem is that any error in phase or frequency of the carrier The problem is that any error in phase or frequency of the carrier at the demodulation results in loss or interference between the two at the demodulation results in loss or interference between the two signals.signals.
Both baseband signals will appear Both baseband signals will appear at the filter output instead of mat the filter output instead of m11(t)(t)
Cochannel interferenceCochannel interference
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DSB has two sidebands, USB and LSB DSB has two sidebands, USB and LSB
SSB has one half of the bandwidth of DSBSSB has one half of the bandwidth of DSB
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SSB (Cont.)SSB (Cont.)
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SSB signal can be coherently (synchronously) demodulated by SSB signal can be coherently (synchronously) demodulated by multiplying it by cos multiplying it by cos ωωcct (exactly like DSB-SC). Example of t (exactly like DSB-SC). Example of
USB demodulation is:USB demodulation is:
SSB-SCSSB-SC
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SSB (Cont.)SSB (Cont.)
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SSB (Cont.)SSB (Cont.)
F -1
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SSB (Cont.)SSB (Cont.)
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SSB (Cont.)SSB (Cont.)Substitute withSubstitute with
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SSB (Cont.)SSB (Cont.)Φ LSB (ω) = M+ ( ω + ωc ) + M- ( ω - ωc )
ΦLSB (t) = m+ (t) e -jωct + m- (t) e jωct
Substitute withSubstitute with
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SSB (Cont.)SSB (Cont.)
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Determination of mDetermination of mhh (t) (t)
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SSB (Cont.)SSB (Cont.)
Hilbert transformHilbert transform
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SSB (Cont.)SSB (Cont.)
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SSB (Cont.)SSB (Cont.)
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SolutionSolution
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Example (Cont.)Example (Cont.)
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Example (Cont.)Example (Cont.)
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DSB is passed through BPF to eliminate undesired bandDSB is passed through BPF to eliminate undesired band
Most commonly usedMost commonly used
To obtain USB, the filter should pass all components To obtain USB, the filter should pass all components above above ωωcc and attenuate all components below and attenuate all components below ωωcc
Difficult to design sharp cutoff filterDifficult to design sharp cutoff filter
Voice spectrumVoice spectrum
300 Hz300 Hz 115
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Generation of SSB signals (Cont.)Generation of SSB signals (Cont.)
Difficult to build116
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Envelope detection of SSB with Envelope detection of SSB with carrier (SSB+C)carrier (SSB+C)
If ‘A’ is large enough If ‘A’ is large enough m(t)m(t) can be recovered by can be recovered by envelop detection envelop detection
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Envelope detection of SSB with carrier Envelope detection of SSB with carrier (SSB+C) (Cont.)(SSB+C) (Cont.)
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Example (Cont.)Example (Cont.)
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SolutionSolution
Diodes conducting Diodes conducting
The diode resistance is ‘ r’ The diode resistance is ‘ r’
off state off state
The diode resistance is ‘ ∞’ The diode resistance is ‘ ∞’
A carrier in positive cycleA carrier in positive cycle
A carrier in negative cycleA carrier in negative cycle
The output voltage is ‘ zero’ The output voltage is ‘ zero’
The diode acts as a gate with gain ‘2R/(R+r)’The diode acts as a gate with gain ‘2R/(R+r)’
(R/(r+R))(R/(r+R))φφ (t) appears across each resistance ‘R’ (t) appears across each resistance ‘R’
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Solution (Cont.)Solution (Cont.) The output is, eThe output is, e00 (t) = (2R/(R+r)) (t) = (2R/(R+r)) ωω (t) m (t) (t) m (t)
If eIf e00 is passed through B.P.F centered is passed through B.P.F centered
at at ωωCC and has 2B band width, the output is and has 2B band width, the output is ( 4R/ ( ( 4R/ ( ππ ( R + r ) ) m ( t ) Cos ( ( R + r ) ) m ( t ) Cos (ωωCC t ) t ) 122
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Solution (Cont.)Solution (Cont.) This circuit can work as a demodulatorThis circuit can work as a demodulatoras follows: as follows:
The input is m (t) cos The input is m (t) cos ωωCC t t
LPF is used at the output LPF is used at the output
The output The output ( 2R/ ( ( 2R/ ( ππ ( R + r ) ) m ( t ) ( R + r ) ) m ( t )
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SolutionSolution
ee00(t) =(t) = ( 4R/ ( ( 4R/ ( ππ ( R + r )) m ( t ) cos ( R + r )) m ( t ) cos ωωCC t t
== k m ( t ) cos k m ( t ) cos ωωCC t t
m ( t )m ( t ) = sin (= sin (ωωCC t + t + θθ ) )
ee00(t)=(t)= k sin ( k sin ( ωωCC t + t + θθ ) cos ) cos ωωCC t t 124
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Solution (Cont.)Solution (Cont.)ee00(t) =(t) = k/2 (sin ( 2 k/2 (sin ( 2 ωωCC t + t + θθ ) + sin ) + sin θθ ) )
The LPF suppresses the sinusoid and transmits The LPF suppresses the sinusoid and transmits only the DC component only the DC component eeoo
//(t) = k/2 sin (t) = k/2 sin θθ
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Phase difference between the two sinusoids
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SolutionSolution
Attenuated by LPF
Removed by DC blocker
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SolutionSolution
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SolutionSolution
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Solution (Cont.)Solution (Cont.)
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Amplitude modulation: Vestigial Amplitude modulation: Vestigial Sideband (VSB)Sideband (VSB)
Due to the difficulties in generating SSB signals
VSB is used
VSB is asymmetric system
VSB is a compromise between DSB and SSB
VSB is easy to generate and its bandwidth is only 25 - 33% greater than SSB
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VSB (Cont.)VSB (Cont.)
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VSB (Cont.)VSB (Cont.)
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VSB (Cont.)VSB (Cont.)
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VSB (Cont.)VSB (Cont.)
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VSB (Cont.)VSB (Cont.)SolutionSolution
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Use of VSB in television broadcastUse of VSB in television broadcast
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Carrier AcquisitionCarrier Acquisition
In any technique from amplitude modulation techniques, the local oscillator must be in synchronous with the oscillator that is used in transmitter side.
Why?Why?Consider DSB-SC case:Consider DSB-SC case:
..
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Carrier Acquisition (Cont.)Carrier Acquisition (Cont.)
.
Filtered by LPFFiltered by LPF.
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Carrier Acquisition (Cont.)Carrier Acquisition (Cont.)
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Attenuation to the messageAttenuation to the message
.
.
Beating effectBeating effect
Solution
Quartz Crystal Oscillator? Difficult to built at high frequencies
Phase Locked Loop (PLL)
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Phase Locked Loop (PLL)Phase Locked Loop (PLL)
Free running frequency Free running frequency ...
Suppressed by LPF Suppressed by LPF
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PLL (Cont.)PLL (Cont.)
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PLL (Cont.)PLL (Cont.)If the frequencies are differentIf the frequencies are different
Suppose the frequency of the input sinusoidalSuppose the frequency of the input sinusoidalsignal is increased from signal is increased from ωωcc to to ωωcc + k + k
This means that the incoming signal isThis means that the incoming signal is
Thus the frequency increase in incoming signal Thus the frequency increase in incoming signal causes causes θθii to increase to to increase to θθii + kt + kt
Increasing Increasing θθe, e, and vice versaand vice versa143
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DC componentDC component
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Carrier Acquisition in DSB-SC Carrier Acquisition in DSB-SC (Cont.)(Cont.)
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Carrier Acquisition in SSB-SC Carrier Acquisition in SSB-SC (Cont.)(Cont.)
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Angle Modulation (Cont.)Angle Modulation (Cont.)
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Angle Modulation (Cont.)Angle Modulation (Cont.) Two techniques are possible to transmit mTwo techniques are possible to transmit m((tt))by varying angle by varying angle θθ of a carrier. of a carrier.
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Angle Modulation (Cont.)Angle Modulation (Cont.)
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Angle Modulation (Cont.)Angle Modulation (Cont.)
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SolutionSolution
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Example (Cont.)Example (Cont.)
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Example (Cont.)Example (Cont.)
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Example (Cont.)Example (Cont.)
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Example (Cont.)Example (Cont.)
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SolutionSolution
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Example (Cont.)Example (Cont.)
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Example (Cont.)Example (Cont.)
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• This scheme is called Frequency Shift Keying (FSK) where information digits are transmitted by keying different frequencies.
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Example (Cont.)Example (Cont.)
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Example (Cont.)Example (Cont.)
• The derivative ṁ(t) is zero except at points of discontinuity of m(t) where impulses of strengths ±2 are present.
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Example (Cont.)Example (Cont.)
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Example (Cont.)Example (Cont.)
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• This scheme is called Phase Shift Keying (PSK) where information digits are transmitted by shifting the carrier phase.
• The phase difference (shift) is π.
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Bandwidth of Angle Modulated Waves (Cont.)Bandwidth of Angle Modulated Waves (Cont.)
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Bandwidth of Angle Modulated Waves (Cont.)Bandwidth of Angle Modulated Waves (Cont.)
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..
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Narrowband Angle Modulation (Cont.)Narrowband Angle Modulation (Cont.)
Compare with AM modulated signalCompare with AM modulated signal
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Generation of NBFMGeneration of NBFM.
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Generation of NBPMGeneration of NBPM
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Can not be ignoredCan not be ignored
Very complicated analysisVery complicated analysis174
In practical FM, In practical FM,
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WBFM (Cont.)WBFM (Cont.)Simple way to analyze the problemSimple way to analyze the problem
m(t)m(t)
m(t) is band limited to B Hzm(t) is band limited to B Hz m(t) is approximated by pulses of constant m(t) is approximated by pulses of constant amplitudes (cells), amplitudes (cells),
FM analysis of constant amplitude is easierFM analysis of constant amplitude is easier
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((StaircaseStaircase))
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WBFM (Cont.)WBFM (Cont.)
m(t) ≈m(t) ≈ Pulse interval ≤ 1 / 2B SecPulse interval ≤ 1 / 2B Sec
The FM signal of one of these cells starting at t = tThe FM signal of one of these cells starting at t = tkk
1/2B Sec1/2B Sec
Sinusoidal Sinusoidal
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WBFM (Cont.)WBFM (Cont.)
The FM spectrum of The FM spectrum of consists of the sumconsists of the sum
of the Fourier transforms of the sinusoidal pulses. of the Fourier transforms of the sinusoidal pulses.
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WBFM (Cont.)WBFM (Cont.)The min. and max. amplitude of modulating signals The min. and max. amplitude of modulating signals are -mare -mpp and m and mpp
the min. frequencythe min. frequency
the max. frequencythe max. frequencythe Bandwidththe Bandwidth
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WBFM (Cont.)WBFM (Cont.).
.
Do not forgetDo not forget, this value is calculated for , this value is calculated for
From earlier analysis:From earlier analysis:
For NBFM, For NBFM,
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The Peak Frequency Deviation:The Peak Frequency Deviation:
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WBFM (Cont.)WBFM (Cont.)
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For a truly wideband case,For a truly wideband case,
A better bandwidth estimate is:A better bandwidth estimate is:
The “deviation ratio” plays a role similar to the The “deviation ratio” plays a role similar to the “modulation index” in AM. “modulation index” in AM.
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SolutionSolution
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Example (Cont.)Example (Cont.)
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Example (Cont.)Example (Cont.)
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Generation of FM WavesGeneration of FM Waves
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• Direct FM Method.
• Indirect FM Method.
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NBFM GenerationNBFM Generation
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Recall:
The output of the this NBFM has some amplitude variations.
A nonlinear device designed to limit the amplitude of a bandpass signal can remove most of this distortion.
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NBFM Generation (Cont.)NBFM Generation (Cont.)
• Bandpass Limiter: used to remove amplitude variations in FM wave.
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• The input-output characteristics of the hard limiter.
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NBFM Generation (Cont.)NBFM Generation (Cont.)
• Hard limiter input and the corresponding output.
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• Hard limiter output with respect to θ
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NBFM Generation (Cont.)NBFM Generation (Cont.)
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NBFM Generation (Cont.)NBFM Generation (Cont.)
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NBFM Generation (Cont.)NBFM Generation (Cont.)
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Demodulation of FM SignalsDemodulation of FM Signals
• The simplest demodulator is an ideal differentiator followed by an envelop detector.
193envelop
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Demodulation of FM Signals (Cont.)Demodulation of FM Signals (Cont.)
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envelop detector can be used to recover m (t )
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FM Demodulation using PLLFM Demodulation using PLL
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Free running frequency Free running frequency