Lecture 7
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COMMUNICATION SYSTEMS Lecture # 7 21st Feb 2007 Instructor
WASEEM KHAN
Centre for Advanced Studies in Engineering
Demodulation of Ordinary AM signal Amplitude (volt)
This process drags a bandpass signal back to baseband.
1.5
The easiest way of demodulation of ordinary AM is envelope detector. At the receiver, the modulated signal can be rectified to extract envelope
1
Message signal
0.5 0
0
0.05 Time (seconds)
0.1
2 Amplitude (volt)
Demodulation is the inverse process of modulation.
1
Modulated carrier and envelope
0 -1 -2
0
0.05 Time (seconds)
0.1
Demodulation of Ordinary AM signal If xAM(t) = [m(t) + A] cos(2 f t + ) then required condition for envelope detection is A | min {m(t)} | The same can be written in terms of modulation index. Modulation index
min m t A
The condition for envelope detection can be formulated as 1 When > 1, the carrier is said to be overmodulated resulting in envelope distortion.
1
Demodulation of DSB-SC AM In case of DSB-SC AM, zero crossing signal reverses the phase of the carrier and envelope detection method is unable to account for it. Messsage signal Modulated carrier
1
Amplitude (volt)
zero Zerocrossing srossing
0
Zero srossing zero crossing -1 0
0.2
0.4
0.6
0.8
1
Time (seconds)
Demodulation of DSB-SC AM Coherent demodulation is required for DSB-SC AM. This involves generation of the replica of carrier frequency at the receiver (note that ordinary AM receiver doesn t generate local carrier signal). The local replica of carrier at receiver must be phase coherent with the carrier generated at the transmitter.
Demodulation of DSB-SC AM A coherent receiver mixes the incoming signal with a locally generated carrier. Multiplying the received signal r(t) by the local carrier yields a signal with a baseband component plus a component at a frequency twice of carrier. Passing this signal through a low pass filter eliminates the high frequency component.
2
Demodulation of DSB-SC AM Mathematical representation The received signal is of the form
r (t )
m(t ) cos( c t
)
When the incoming signal is multiplied by local carrier signal, we get
r (t ) cos(
c
t
)
m (t ) cos(
c
m (t ) cos( 2 2
t
) cos( ct
If the two carrier signals are synchronized, i.e.
r (t ) cos( c t
)
c
t
)
) cos(
)
= , then
m (t ) cos(2 c t 2 ) cos(0) 2
m (t ) 2
filtered by LPF
SSB AM can also be demodulated using the same technique.
Phase and Frequency Relation cos( t + ) Initial phase or angle
Instantaneous phase or angle
Instantaneous frequency
d (instantaneous phase) dt
Angle modulation Angle modulation is the process by which the phase angle of a carrier is varied according to the message signal. Angle modulation is sub-classified into frequency modulation (FM) and phase modulation (PM). In PM, the instantaneous phase deviation of the carrier is proportional to the message signal. In FM, the instantaneous frequency deviation of the carrier is proportional to the message signal.
3
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WASEEM KHAN
Centre for Advanced Studies in Engineering
Demodulation of Ordinary AM signal Amplitude (volt)
This process drags a bandpass signal back to baseband.
1.5
The easiest way of demodulation of ordinary AM is envelope detector. At the receiver, the modulated signal can be rectified to extract envelope
1
Message signal
0.5 0
0
0.05 Time (seconds)
0.1
2 Amplitude (volt)
Demodulation is the inverse process of modulation.
1
Modulated carrier and envelope
0 -1 -2
0
0.05 Time (seconds)
0.1
Demodulation of Ordinary AM signal If xAM(t) = [m(t) + A] cos(2 f t + ) then required condition for envelope detection is A | min {m(t)} | The same can be written in terms of modulation index. Modulation index
min m t A
The condition for envelope detection can be formulated as 1 When > 1, the carrier is said to be overmodulated resulting in envelope distortion.
1
Demodulation of DSB-SC AM In case of DSB-SC AM, zero crossing signal reverses the phase of the carrier and envelope detection method is unable to account for it. Messsage signal Modulated carrier
1
Amplitude (volt)
zero Zerocrossing srossing
0
Zero srossing zero crossing -1 0
0.2
0.4
0.6
0.8
1
Time (seconds)
Demodulation of DSB-SC AM Coherent demodulation is required for DSB-SC AM. This involves generation of the replica of carrier frequency at the receiver (note that ordinary AM receiver doesn t generate local carrier signal). The local replica of carrier at receiver must be phase coherent with the carrier generated at the transmitter.
Demodulation of DSB-SC AM A coherent receiver mixes the incoming signal with a locally generated carrier. Multiplying the received signal r(t) by the local carrier yields a signal with a baseband component plus a component at a frequency twice of carrier. Passing this signal through a low pass filter eliminates the high frequency component.
2
Demodulation of DSB-SC AM Mathematical representation The received signal is of the form
r (t )
m(t ) cos( c t
)
When the incoming signal is multiplied by local carrier signal, we get
r (t ) cos(
c
t
)
m (t ) cos(
c
m (t ) cos( 2 2
t
) cos( ct
If the two carrier signals are synchronized, i.e.
r (t ) cos( c t
)
c
t
)
) cos(
)
= , then
m (t ) cos(2 c t 2 ) cos(0) 2
m (t ) 2
filtered by LPF
SSB AM can also be demodulated using the same technique.
Phase and Frequency Relation cos( t + ) Initial phase or angle
Instantaneous phase or angle
Instantaneous frequency
d (instantaneous phase) dt
Angle modulation Angle modulation is the process by which the phase angle of a carrier is varied according to the message signal. Angle modulation is sub-classified into frequency modulation (FM) and phase modulation (PM). In PM, the instantaneous phase deviation of the carrier is proportional to the message signal. In FM, the instantaneous frequency deviation of the carrier is proportional to the message signal.
3
This document was created with Win2PDF available at http://www.daneprairie.com. The unregistered version of Win2PDF is for evaluation or non-commercial use only.
