Digital Modulation Techniques & Data Transmission | Communication System - Electronics and Communication Engineering (ECE) PDF Download

 Page 1


1 
 
 
 
DIGITAL MODULATION TECHNIQUES 
 
Digital Modulation provides more information capacity, high data security, quicker system 
availability with great quality communication. Hence, digital modulation techniques have a greater 
demand, for their capacity to convey larger amounts of data than analog ones. 
There are many types of digital modulation techniques and we can even use a combination of these 
techniques as well. In this chapter, we will be discussing the most prominent digital modulation 
techniques. 
if the information signal is digital and the amplitude (lV of the carrier is varied proportional to 
the information signal, a digitally modulated signal called amplitude shift keying (ASK) is 
produced. 
If the frequency (f) is varied proportional to the information signal, frequency shift keying (FSK) is 
produced, and if the phase of the carrier (0) is varied proportional to the information signal, 
phase shift keying (PSK) is produced. If both the amplitude and the phase are varied proportional to 
the information signal, quadrature amplitude modulation (QAM) results. ASK, FSK, PSK, and 
QAM are all forms of digital modulation: 
a simplified block diagram for a digital modulation system. 
 
 
Amplitude Shift Keying 
The amplitude of the resultant output depends upon the input data whether it should be a zero level 
or a variation of positive and negative, depending upon the carrier frequency. 
Amplitude Shift Keying (ASK) is a type of Amplitude Modulation which represents the binary 
data in the form of variations in the amplitude of a signal. 
Following is the diagram for ASK modulated waveform along with its input. 
Page 2


1 
 
 
 
DIGITAL MODULATION TECHNIQUES 
 
Digital Modulation provides more information capacity, high data security, quicker system 
availability with great quality communication. Hence, digital modulation techniques have a greater 
demand, for their capacity to convey larger amounts of data than analog ones. 
There are many types of digital modulation techniques and we can even use a combination of these 
techniques as well. In this chapter, we will be discussing the most prominent digital modulation 
techniques. 
if the information signal is digital and the amplitude (lV of the carrier is varied proportional to 
the information signal, a digitally modulated signal called amplitude shift keying (ASK) is 
produced. 
If the frequency (f) is varied proportional to the information signal, frequency shift keying (FSK) is 
produced, and if the phase of the carrier (0) is varied proportional to the information signal, 
phase shift keying (PSK) is produced. If both the amplitude and the phase are varied proportional to 
the information signal, quadrature amplitude modulation (QAM) results. ASK, FSK, PSK, and 
QAM are all forms of digital modulation: 
a simplified block diagram for a digital modulation system. 
 
 
Amplitude Shift Keying 
The amplitude of the resultant output depends upon the input data whether it should be a zero level 
or a variation of positive and negative, depending upon the carrier frequency. 
Amplitude Shift Keying (ASK) is a type of Amplitude Modulation which represents the binary 
data in the form of variations in the amplitude of a signal. 
Following is the diagram for ASK modulated waveform along with its input. 
2 
 
 
 
 
Any modulated signal has a high frequency carrier. The binary signal when ASK is modulated, 
gives a zero value for LOW input and gives the carrier output for HIGH input. 
Mathematically, amplitude-shift keying is 
 
 
where vask(t) = amplitude-shift keying wave 
vm(t) = digital information (modulating) signal (volts) 
A/2 = unmodulated carrier amplitude (volts) 
?c= analog carrier radian frequency (radians per second, 2pfct) 
 
 
In above Equation, the modulating signal [vm(t)] is a normalized binary waveform, where + 1 V = 
logic 1 and -1 V = logic 0. Therefore, for a logic 1 input, vm(t) = + 1 V, Equation 2.12 reduces to 
 
 
 
Mathematically, amplitude-shift keying is (2.12) where vask(t) = amplitude-shift keying wave  
vm(t) = digital information (modulating) signal (volts) A/2 = unmodulated carrier amplitude (volts) 
Page 3


1 
 
 
 
DIGITAL MODULATION TECHNIQUES 
 
Digital Modulation provides more information capacity, high data security, quicker system 
availability with great quality communication. Hence, digital modulation techniques have a greater 
demand, for their capacity to convey larger amounts of data than analog ones. 
There are many types of digital modulation techniques and we can even use a combination of these 
techniques as well. In this chapter, we will be discussing the most prominent digital modulation 
techniques. 
if the information signal is digital and the amplitude (lV of the carrier is varied proportional to 
the information signal, a digitally modulated signal called amplitude shift keying (ASK) is 
produced. 
If the frequency (f) is varied proportional to the information signal, frequency shift keying (FSK) is 
produced, and if the phase of the carrier (0) is varied proportional to the information signal, 
phase shift keying (PSK) is produced. If both the amplitude and the phase are varied proportional to 
the information signal, quadrature amplitude modulation (QAM) results. ASK, FSK, PSK, and 
QAM are all forms of digital modulation: 
a simplified block diagram for a digital modulation system. 
 
 
Amplitude Shift Keying 
The amplitude of the resultant output depends upon the input data whether it should be a zero level 
or a variation of positive and negative, depending upon the carrier frequency. 
Amplitude Shift Keying (ASK) is a type of Amplitude Modulation which represents the binary 
data in the form of variations in the amplitude of a signal. 
Following is the diagram for ASK modulated waveform along with its input. 
2 
 
 
 
 
Any modulated signal has a high frequency carrier. The binary signal when ASK is modulated, 
gives a zero value for LOW input and gives the carrier output for HIGH input. 
Mathematically, amplitude-shift keying is 
 
 
where vask(t) = amplitude-shift keying wave 
vm(t) = digital information (modulating) signal (volts) 
A/2 = unmodulated carrier amplitude (volts) 
?c= analog carrier radian frequency (radians per second, 2pfct) 
 
 
In above Equation, the modulating signal [vm(t)] is a normalized binary waveform, where + 1 V = 
logic 1 and -1 V = logic 0. Therefore, for a logic 1 input, vm(t) = + 1 V, Equation 2.12 reduces to 
 
 
 
Mathematically, amplitude-shift keying is (2.12) where vask(t) = amplitude-shift keying wave  
vm(t) = digital information (modulating) signal (volts) A/2 = unmodulated carrier amplitude (volts) 
3 
 
?c= analog carrier radian frequency (radians per second, 2pfct) In Equation 2.12, the modulating 
signal [vm(t)] is a normalized binary waveform, where + 1 V = logic 1 and -1 V = logic 0. 
Therefore, for a logic 1 input, vm(t) = + 1 V, Equation 2.12 reduces to and for a logic 0 input, vm(t) 
= -1 V,Equation reduces to 
 
Thus, the modulated wave vask(t),is either A cos(?ct) or 0. Hence, the carrier is either "on “or 
"off," which is why amplitude-shift keying is sometimes referred to as on-off keying (OOK). 
it can be seen that for every change in the input binary data stream, there is one change in the ASK 
waveform, and the time of one bit (tb) equals the time of one analog signaling element (t,). 
B = fb/1 = fb baud = fb/1 = fb 
 
Example : 
Determine the baud and minimum bandwidth necessary to pass a 10 kbps binary signal using 
amplitude shift keying. 10Solution For ASK, N = 1, and the baud and minimum bandwidth are 
determined from Equations 2.11 and 2.10, respectively: 
 
B = 10,000 / 1 = 10,000 
baud = 10, 000 /1 = 10,000 
The use of amplitude-modulated analog carriers to transport digital information is a relatively low- 
quality, low-cost type of digital modulation and, therefore, is seldom used except for very low- 
speed telemetry circuits. 
ASK TRANSMITTER: 
 
Page 4


1 
 
 
 
DIGITAL MODULATION TECHNIQUES 
 
Digital Modulation provides more information capacity, high data security, quicker system 
availability with great quality communication. Hence, digital modulation techniques have a greater 
demand, for their capacity to convey larger amounts of data than analog ones. 
There are many types of digital modulation techniques and we can even use a combination of these 
techniques as well. In this chapter, we will be discussing the most prominent digital modulation 
techniques. 
if the information signal is digital and the amplitude (lV of the carrier is varied proportional to 
the information signal, a digitally modulated signal called amplitude shift keying (ASK) is 
produced. 
If the frequency (f) is varied proportional to the information signal, frequency shift keying (FSK) is 
produced, and if the phase of the carrier (0) is varied proportional to the information signal, 
phase shift keying (PSK) is produced. If both the amplitude and the phase are varied proportional to 
the information signal, quadrature amplitude modulation (QAM) results. ASK, FSK, PSK, and 
QAM are all forms of digital modulation: 
a simplified block diagram for a digital modulation system. 
 
 
Amplitude Shift Keying 
The amplitude of the resultant output depends upon the input data whether it should be a zero level 
or a variation of positive and negative, depending upon the carrier frequency. 
Amplitude Shift Keying (ASK) is a type of Amplitude Modulation which represents the binary 
data in the form of variations in the amplitude of a signal. 
Following is the diagram for ASK modulated waveform along with its input. 
2 
 
 
 
 
Any modulated signal has a high frequency carrier. The binary signal when ASK is modulated, 
gives a zero value for LOW input and gives the carrier output for HIGH input. 
Mathematically, amplitude-shift keying is 
 
 
where vask(t) = amplitude-shift keying wave 
vm(t) = digital information (modulating) signal (volts) 
A/2 = unmodulated carrier amplitude (volts) 
?c= analog carrier radian frequency (radians per second, 2pfct) 
 
 
In above Equation, the modulating signal [vm(t)] is a normalized binary waveform, where + 1 V = 
logic 1 and -1 V = logic 0. Therefore, for a logic 1 input, vm(t) = + 1 V, Equation 2.12 reduces to 
 
 
 
Mathematically, amplitude-shift keying is (2.12) where vask(t) = amplitude-shift keying wave  
vm(t) = digital information (modulating) signal (volts) A/2 = unmodulated carrier amplitude (volts) 
3 
 
?c= analog carrier radian frequency (radians per second, 2pfct) In Equation 2.12, the modulating 
signal [vm(t)] is a normalized binary waveform, where + 1 V = logic 1 and -1 V = logic 0. 
Therefore, for a logic 1 input, vm(t) = + 1 V, Equation 2.12 reduces to and for a logic 0 input, vm(t) 
= -1 V,Equation reduces to 
 
Thus, the modulated wave vask(t),is either A cos(?ct) or 0. Hence, the carrier is either "on “or 
"off," which is why amplitude-shift keying is sometimes referred to as on-off keying (OOK). 
it can be seen that for every change in the input binary data stream, there is one change in the ASK 
waveform, and the time of one bit (tb) equals the time of one analog signaling element (t,). 
B = fb/1 = fb baud = fb/1 = fb 
 
Example : 
Determine the baud and minimum bandwidth necessary to pass a 10 kbps binary signal using 
amplitude shift keying. 10Solution For ASK, N = 1, and the baud and minimum bandwidth are 
determined from Equations 2.11 and 2.10, respectively: 
 
B = 10,000 / 1 = 10,000 
baud = 10, 000 /1 = 10,000 
The use of amplitude-modulated analog carriers to transport digital information is a relatively low- 
quality, low-cost type of digital modulation and, therefore, is seldom used except for very low- 
speed telemetry circuits. 
ASK TRANSMITTER: 
 
4 
 
The input binary sequence is applied to the product modulator. The product modulator amplitude 
modulates the sinusoidal carrier .it passes the carrier when input bit is ‘1’ .it blocks the carrier when 
input bit is ‘0.’ 
Coherent ASK DETECTOR: 
 
FREQUENCYSHIFT KEYING 
The frequency of the output signal will be either high or low, depending upon the input data 
applied. 
Frequency Shift Keying (FSK) is the digital modulation technique in which the frequency of the 
carrier signal varies according to the discrete digital changes. FSK is a scheme of frequency 
modulation. 
Following is the diagram for FSK modulated waveform along with its input. 
 
 
 
 
The output of a FSK modulated wave is high in frequency for a binary HIGH input and is low in 
frequency for a binary LOW input. The binary 1s and 0s are called Mark and Space frequencies. 
FSK is a form of constant-amplitude angle modulation similar to standard frequency modulation 
(FM) except the modulating signal is a binary signal that varies between two discrete voltage levels 
rather than a continuously changing analog waveform.Consequently, FSK is sometimes called 
binary FSK (BFSK). The general expression for FSK is 
Page 5


1 
 
 
 
DIGITAL MODULATION TECHNIQUES 
 
Digital Modulation provides more information capacity, high data security, quicker system 
availability with great quality communication. Hence, digital modulation techniques have a greater 
demand, for their capacity to convey larger amounts of data than analog ones. 
There are many types of digital modulation techniques and we can even use a combination of these 
techniques as well. In this chapter, we will be discussing the most prominent digital modulation 
techniques. 
if the information signal is digital and the amplitude (lV of the carrier is varied proportional to 
the information signal, a digitally modulated signal called amplitude shift keying (ASK) is 
produced. 
If the frequency (f) is varied proportional to the information signal, frequency shift keying (FSK) is 
produced, and if the phase of the carrier (0) is varied proportional to the information signal, 
phase shift keying (PSK) is produced. If both the amplitude and the phase are varied proportional to 
the information signal, quadrature amplitude modulation (QAM) results. ASK, FSK, PSK, and 
QAM are all forms of digital modulation: 
a simplified block diagram for a digital modulation system. 
 
 
Amplitude Shift Keying 
The amplitude of the resultant output depends upon the input data whether it should be a zero level 
or a variation of positive and negative, depending upon the carrier frequency. 
Amplitude Shift Keying (ASK) is a type of Amplitude Modulation which represents the binary 
data in the form of variations in the amplitude of a signal. 
Following is the diagram for ASK modulated waveform along with its input. 
2 
 
 
 
 
Any modulated signal has a high frequency carrier. The binary signal when ASK is modulated, 
gives a zero value for LOW input and gives the carrier output for HIGH input. 
Mathematically, amplitude-shift keying is 
 
 
where vask(t) = amplitude-shift keying wave 
vm(t) = digital information (modulating) signal (volts) 
A/2 = unmodulated carrier amplitude (volts) 
?c= analog carrier radian frequency (radians per second, 2pfct) 
 
 
In above Equation, the modulating signal [vm(t)] is a normalized binary waveform, where + 1 V = 
logic 1 and -1 V = logic 0. Therefore, for a logic 1 input, vm(t) = + 1 V, Equation 2.12 reduces to 
 
 
 
Mathematically, amplitude-shift keying is (2.12) where vask(t) = amplitude-shift keying wave  
vm(t) = digital information (modulating) signal (volts) A/2 = unmodulated carrier amplitude (volts) 
3 
 
?c= analog carrier radian frequency (radians per second, 2pfct) In Equation 2.12, the modulating 
signal [vm(t)] is a normalized binary waveform, where + 1 V = logic 1 and -1 V = logic 0. 
Therefore, for a logic 1 input, vm(t) = + 1 V, Equation 2.12 reduces to and for a logic 0 input, vm(t) 
= -1 V,Equation reduces to 
 
Thus, the modulated wave vask(t),is either A cos(?ct) or 0. Hence, the carrier is either "on “or 
"off," which is why amplitude-shift keying is sometimes referred to as on-off keying (OOK). 
it can be seen that for every change in the input binary data stream, there is one change in the ASK 
waveform, and the time of one bit (tb) equals the time of one analog signaling element (t,). 
B = fb/1 = fb baud = fb/1 = fb 
 
Example : 
Determine the baud and minimum bandwidth necessary to pass a 10 kbps binary signal using 
amplitude shift keying. 10Solution For ASK, N = 1, and the baud and minimum bandwidth are 
determined from Equations 2.11 and 2.10, respectively: 
 
B = 10,000 / 1 = 10,000 
baud = 10, 000 /1 = 10,000 
The use of amplitude-modulated analog carriers to transport digital information is a relatively low- 
quality, low-cost type of digital modulation and, therefore, is seldom used except for very low- 
speed telemetry circuits. 
ASK TRANSMITTER: 
 
4 
 
The input binary sequence is applied to the product modulator. The product modulator amplitude 
modulates the sinusoidal carrier .it passes the carrier when input bit is ‘1’ .it blocks the carrier when 
input bit is ‘0.’ 
Coherent ASK DETECTOR: 
 
FREQUENCYSHIFT KEYING 
The frequency of the output signal will be either high or low, depending upon the input data 
applied. 
Frequency Shift Keying (FSK) is the digital modulation technique in which the frequency of the 
carrier signal varies according to the discrete digital changes. FSK is a scheme of frequency 
modulation. 
Following is the diagram for FSK modulated waveform along with its input. 
 
 
 
 
The output of a FSK modulated wave is high in frequency for a binary HIGH input and is low in 
frequency for a binary LOW input. The binary 1s and 0s are called Mark and Space frequencies. 
FSK is a form of constant-amplitude angle modulation similar to standard frequency modulation 
(FM) except the modulating signal is a binary signal that varies between two discrete voltage levels 
rather than a continuously changing analog waveform.Consequently, FSK is sometimes called 
binary FSK (BFSK). The general expression for FSK is 
5 
 
 
 
where 
 
vfsk(t) = binary FSK waveform 
 
Vc  = peak analog carrier amplitude (volts) 
 
fc  = analog carrier center frequency (hertz) 
 
f=peak change (shift)in the analog carrier frequency(hertz) 
vm(t) = binary input (modulating) signal (volts) 
From Equation 2.13, it can be seen that the peak shift in the carrier frequency ( f) is proportional to 
the amplitude of the binary input signal (v m[t]), and the direction of the shift is determined by the 
polarity. 
The modulating signal is a normalized binary waveform where a logic 1 = + 1 V and a logic 0 = -1 
V. Thus, for a logic l input, vm(t) = + 1, Equation 2.13 can be rewritten as 
For a logic 0 input, vm(t) = -1, Equation becomes 
 
 
With binary FSK, the carrier center frequency (fc) is shifted (deviated) up and down in the 
frequency domain by the binary input signal as shown in Figure 2-3. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
FIGURE: FSK in the frequency domain