Analog Communication Formulas for GATE ECE Exam | Communication System - Electronics and Communication Engineering (ECE) PDF Download

 Page 1


 
 
 
                                                                            
 
ANALOG COMMUNICATION 
 
Modulation  
It is the process of placing the message signal over some carrier signal to make it suitable for transmission.  
 
Need for Modulation:  
1. Size of antenna required for receiving the wave is reduced if signal is transmitted at high frequency.  
2. Many number of signals can be transmitted simultaneously by selecting the carriers of different frequencies.  
3. The interference of noise and other signals can be reduced by changing the frequency of transmission.  
4. Integration of different communication system is possible. 
 
Amplitude Modulation 
Amplitude Modulated Signal:  
AM may be defined as a system in which the maximum amplitude of the carrier wave is made proportional to 
the instantaneous value (amplitude) of the modulating or base band signal 
 
 
Frequency spectrum of AM wave: 
 
 
Amin – minimum amplitude 
Page 2


 
 
 
                                                                            
 
ANALOG COMMUNICATION 
 
Modulation  
It is the process of placing the message signal over some carrier signal to make it suitable for transmission.  
 
Need for Modulation:  
1. Size of antenna required for receiving the wave is reduced if signal is transmitted at high frequency.  
2. Many number of signals can be transmitted simultaneously by selecting the carriers of different frequencies.  
3. The interference of noise and other signals can be reduced by changing the frequency of transmission.  
4. Integration of different communication system is possible. 
 
Amplitude Modulation 
Amplitude Modulated Signal:  
AM may be defined as a system in which the maximum amplitude of the carrier wave is made proportional to 
the instantaneous value (amplitude) of the modulating or base band signal 
 
 
Frequency spectrum of AM wave: 
 
 
Amin – minimum amplitude 
 
 
 
                                                                            
 
Power Relations in AM wave: 
 
Maximum power dissipated in AM wave is PAM= 1.5 Pc for (not=) 1 and this is maximum power that amplifier 
can handle without distortion 
 
Efficiency of Amplitude Modulated System: 
 
Multi-tone Modulation:  
When carrier is modulated simultaneously by more than one sinusoidal signal.  
Resultant Modulation Index   
 
Double side Band Suppressed Carrier modulation DSB-SC: 
 
In DSB-SC the carrier signal is suppressed at the time of modulation. Only side- bands are transmitted in 
modulated wave. 
 
 
Single Sideband Modulation (SSB):  
In this technique, along with modulation carrier one side band gets suppressed from AM modulated wave. 
Page 3


 
 
 
                                                                            
 
ANALOG COMMUNICATION 
 
Modulation  
It is the process of placing the message signal over some carrier signal to make it suitable for transmission.  
 
Need for Modulation:  
1. Size of antenna required for receiving the wave is reduced if signal is transmitted at high frequency.  
2. Many number of signals can be transmitted simultaneously by selecting the carriers of different frequencies.  
3. The interference of noise and other signals can be reduced by changing the frequency of transmission.  
4. Integration of different communication system is possible. 
 
Amplitude Modulation 
Amplitude Modulated Signal:  
AM may be defined as a system in which the maximum amplitude of the carrier wave is made proportional to 
the instantaneous value (amplitude) of the modulating or base band signal 
 
 
Frequency spectrum of AM wave: 
 
 
Amin – minimum amplitude 
 
 
 
                                                                            
 
Power Relations in AM wave: 
 
Maximum power dissipated in AM wave is PAM= 1.5 Pc for (not=) 1 and this is maximum power that amplifier 
can handle without distortion 
 
Efficiency of Amplitude Modulated System: 
 
Multi-tone Modulation:  
When carrier is modulated simultaneously by more than one sinusoidal signal.  
Resultant Modulation Index   
 
Double side Band Suppressed Carrier modulation DSB-SC: 
 
In DSB-SC the carrier signal is suppressed at the time of modulation. Only side- bands are transmitted in 
modulated wave. 
 
 
Single Sideband Modulation (SSB):  
In this technique, along with modulation carrier one side band gets suppressed from AM modulated wave. 
 
 
 
                                                                            
 
 
 
Vestigial Sideband (VSB) modulation:  
In this modulation one side band and vestige of another sideband is transmitted.  
• It is used for transmission of video signal in television broadcasting 
• It is also used for high speed data signal and facsimile.  
• Vocal signal transmission of T.V. via F.M 
 
AM Modulators:  
For Generation of AM or DSB/Full carrier wave  
A. Product Modulator  
B. Square Law Modulator  
C. Switching Modulator 
For Generation DSB-SC wave  
A. Filter method/frequency discrimination method  
B. Phase shift method/Phase discrimination method  
C. Third method/Weaver’s method 
 
Demodulation of Amplitude Modulate wave: 
A. Synchronous or coherent detection  
B. Envelop detector Envelop Detector: 
 
r(t) is received signal and m(t) is message signal and for better reception RC must be selected such as 
 
 
 
Page 4


 
 
 
                                                                            
 
ANALOG COMMUNICATION 
 
Modulation  
It is the process of placing the message signal over some carrier signal to make it suitable for transmission.  
 
Need for Modulation:  
1. Size of antenna required for receiving the wave is reduced if signal is transmitted at high frequency.  
2. Many number of signals can be transmitted simultaneously by selecting the carriers of different frequencies.  
3. The interference of noise and other signals can be reduced by changing the frequency of transmission.  
4. Integration of different communication system is possible. 
 
Amplitude Modulation 
Amplitude Modulated Signal:  
AM may be defined as a system in which the maximum amplitude of the carrier wave is made proportional to 
the instantaneous value (amplitude) of the modulating or base band signal 
 
 
Frequency spectrum of AM wave: 
 
 
Amin – minimum amplitude 
 
 
 
                                                                            
 
Power Relations in AM wave: 
 
Maximum power dissipated in AM wave is PAM= 1.5 Pc for (not=) 1 and this is maximum power that amplifier 
can handle without distortion 
 
Efficiency of Amplitude Modulated System: 
 
Multi-tone Modulation:  
When carrier is modulated simultaneously by more than one sinusoidal signal.  
Resultant Modulation Index   
 
Double side Band Suppressed Carrier modulation DSB-SC: 
 
In DSB-SC the carrier signal is suppressed at the time of modulation. Only side- bands are transmitted in 
modulated wave. 
 
 
Single Sideband Modulation (SSB):  
In this technique, along with modulation carrier one side band gets suppressed from AM modulated wave. 
 
 
 
                                                                            
 
 
 
Vestigial Sideband (VSB) modulation:  
In this modulation one side band and vestige of another sideband is transmitted.  
• It is used for transmission of video signal in television broadcasting 
• It is also used for high speed data signal and facsimile.  
• Vocal signal transmission of T.V. via F.M 
 
AM Modulators:  
For Generation of AM or DSB/Full carrier wave  
A. Product Modulator  
B. Square Law Modulator  
C. Switching Modulator 
For Generation DSB-SC wave  
A. Filter method/frequency discrimination method  
B. Phase shift method/Phase discrimination method  
C. Third method/Weaver’s method 
 
Demodulation of Amplitude Modulate wave: 
A. Synchronous or coherent detection  
B. Envelop detector Envelop Detector: 
 
r(t) is received signal and m(t) is message signal and for better reception RC must be selected such as 
 
 
 
 
 
 
                                                                            
 
Key points:  
Demodulation of AM signal is simpler than DSB-SC and SSB systems, Demodulation of DSB-SC and SSB is 
rather difficult and expensive.  
It is quite easier to generate conventional AM signals at high power level as compared to DSB-SC and SSB 
signals. For this reason, conventional AM systems are used for broad casting purpose.  
The advantage of DSB-SC and SSB systems over conventional AM system is that the former requires lesser 
power to transmit the same information.  
SSB scheme needs only one half of the bandwidth required in DSB-SC system and less than that required in 
VSB also.  
SSB modulation scheme is used for long distance transmission of voice signals because it allows longer spacing 
between repeaters 
 
 
***