`color{blue} ✍️`As already mentioned, the purpose of a communication system is to transmit information or message signals.
Message signals are also called `"baseband signals,"` which essentially designate the band of frequencies representing the original signal, as delivered by the source of information.
`color{blue} ✍️`No signal, in general, is a single frequency sinusoid, but it spreads over a range of frequencies called the signal bandwidth.
`color{blue} ✍️`Suppose we wish to transmit an electronic signal in the audio frequency (AF) range (baseband signal frequency less than 20 kHz) over a long distance directly. Let us find what factors prevent us from doing so and how we overcome these factors.
`color{brown}bbul("Size of the antenna or aerial")`
`color{blue} ✍️`For transmitting a signal, we need an antenna or an aerial. This antenna should have a size comparable to the wavelength of the signal (at least `lamda//4` in dimension) so that the antenna properly senses the time variation of the signal.
`color{blue} ✍️`For an electromagnetic wave of frequency 20 kHz, the wavelength `lamda` is 15 km. Obviously, such a long antenna is not possible to construct and operate. Hence direct transmission of such baseband signals is not practical.
`color{blue} ✍️`We can obtain transmission with reasonable antenna lengths if transmission frequency is high (for example, if n is 1 MHz, then `lamda` is 300 m).
`color{blue} ✍️`Therefore, there is a need of translating the information contained in our original low frequency baseband signal into high or radio frequencies before transmission.
`color{brown}bbul("Effective power radiated by an antenna")`
`color{blue} ✍️`A theoretical study of radiation from a linear antenna (length l) shows that the power radiated is proportional to `(lamda//l)^2` .
`color{blue} ✍️`This implies that for the same antenna length, the power radiated increases with decreasing l, i.e., increasing frequency.
`color{blue} ✍️`Hence, the effective power radiated by a long wavelength baseband signal would be small. For a good transmission, we need high powers and hence this also points out to the need of using high frequency transmission.
`color{brown}bbul("Mixing up of signals from different transmitters")`
`color{blue} ✍️`Another important argument against transmitting baseband signals directly is more practical in nature.
Suppose many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously. All these signals will get mixed up and there is no simple way to distinguish between them.
`color{blue} ✍️`This points out towards a possible solution by using communication at high frequencies and allotting a band of frequencies to each message signal for its transmission.
`color{blue} ✍️`The above arguments suggest that there is a need for translating the original low frequency baseband message or information signal into high frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal.
`color{blue} ✍️` In doing so, we take the help of a high frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it. The carrier wave may be continuous (sinusoidal) or in the form of pulses as shown in Fig. 15.7.
`color{blue} ✍️`A sinusoidal carrier wave can be represented as
`color{blue}(c(t)=A_csin(omega_ct+phi))`
............(15.2)
`color{blue} ✍️`where `c(t)` is the signal strength (voltage or current), `A_c` is the amplitude, `omega_c` ( = 2pnc) is the angular frequency and `f` is the initial phase of the carrier wave.
`color{blue} ✍️`During the process of modulation, any of the three parameters, viz `A_c, omega_c` and `phi`, of the carrier wave can be controlled by the message or information signal.
`color{blue} ✍️`This results in three types of modulation: (i) Amplitude modulation (AM), (ii) Frequency modulation (FM) and (iii) Phase modulation (PM), as shown in Fig. 15.8.
`color{blue} ✍️`Similarly, the significant characteristics of a pulse are: pulse amplitude, pulse duration or pulse Width, and pulse position (denoting the time of rise or fall of the pulse amplitude) as shown in Fig. 15.7(b).
`color{blue} ✍️`Hence, different types of pulse modulation are: (a) pulse amplitude modulation (PAM), (b) pulse duration modulation (PDM) or pulse width modulation (PWM), and (c) pulse position modulation (PPM). In this chapter, we shall confine to amplitude modulation on ly.
`color{blue} ✍️`As already mentioned, the purpose of a communication system is to transmit information or message signals.
Message signals are also called `"baseband signals,"` which essentially designate the band of frequencies representing the original signal, as delivered by the source of information.
`color{blue} ✍️`No signal, in general, is a single frequency sinusoid, but it spreads over a range of frequencies called the signal bandwidth.
`color{blue} ✍️`Suppose we wish to transmit an electronic signal in the audio frequency (AF) range (baseband signal frequency less than 20 kHz) over a long distance directly. Let us find what factors prevent us from doing so and how we overcome these factors.
`color{brown}bbul("Size of the antenna or aerial")`
`color{blue} ✍️`For transmitting a signal, we need an antenna or an aerial. This antenna should have a size comparable to the wavelength of the signal (at least `lamda//4` in dimension) so that the antenna properly senses the time variation of the signal.
`color{blue} ✍️`For an electromagnetic wave of frequency 20 kHz, the wavelength `lamda` is 15 km. Obviously, such a long antenna is not possible to construct and operate. Hence direct transmission of such baseband signals is not practical.
`color{blue} ✍️`We can obtain transmission with reasonable antenna lengths if transmission frequency is high (for example, if n is 1 MHz, then `lamda` is 300 m).
`color{blue} ✍️`Therefore, there is a need of translating the information contained in our original low frequency baseband signal into high or radio frequencies before transmission.
`color{brown}bbul("Effective power radiated by an antenna")`
`color{blue} ✍️`A theoretical study of radiation from a linear antenna (length l) shows that the power radiated is proportional to `(lamda//l)^2` .
`color{blue} ✍️`This implies that for the same antenna length, the power radiated increases with decreasing l, i.e., increasing frequency.
`color{blue} ✍️`Hence, the effective power radiated by a long wavelength baseband signal would be small. For a good transmission, we need high powers and hence this also points out to the need of using high frequency transmission.
`color{brown}bbul("Mixing up of signals from different transmitters")`
`color{blue} ✍️`Another important argument against transmitting baseband signals directly is more practical in nature.
Suppose many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously. All these signals will get mixed up and there is no simple way to distinguish between them.
`color{blue} ✍️`This points out towards a possible solution by using communication at high frequencies and allotting a band of frequencies to each message signal for its transmission.
`color{blue} ✍️`The above arguments suggest that there is a need for translating the original low frequency baseband message or information signal into high frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal.
`color{blue} ✍️` In doing so, we take the help of a high frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it. The carrier wave may be continuous (sinusoidal) or in the form of pulses as shown in Fig. 15.7.
`color{blue} ✍️`A sinusoidal carrier wave can be represented as
`color{blue}(c(t)=A_csin(omega_ct+phi))`
............(15.2)
`color{blue} ✍️`where `c(t)` is the signal strength (voltage or current), `A_c` is the amplitude, `omega_c` ( = 2pnc) is the angular frequency and `f` is the initial phase of the carrier wave.
`color{blue} ✍️`During the process of modulation, any of the three parameters, viz `A_c, omega_c` and `phi`, of the carrier wave can be controlled by the message or information signal.
`color{blue} ✍️`This results in three types of modulation: (i) Amplitude modulation (AM), (ii) Frequency modulation (FM) and (iii) Phase modulation (PM), as shown in Fig. 15.8.
`color{blue} ✍️`Similarly, the significant characteristics of a pulse are: pulse amplitude, pulse duration or pulse Width, and pulse position (denoting the time of rise or fall of the pulse amplitude) as shown in Fig. 15.7(b).
`color{blue} ✍️`Hence, different types of pulse modulation are: (a) pulse amplitude modulation (PAM), (b) pulse duration modulation (PDM) or pulse width modulation (PWM), and (c) pulse position modulation (PPM). In this chapter, we shall confine to amplitude modulation on ly.