WBCHSE Class 12 Physics Communication System Notes

Communication System Introduction

The word ‘communication’ has a widespread meaning in rawenGH our daily lives, as expanded by the modem technology. It means the transmission of information or ideas using words and gestures, books and periodicals, radio-television, video, mobile, telephone, and internet. The importance of the communication system as exemplified by the above can hardly be exaggerated. For every communication system, there are two ends called transmitter and receiver. The matter communicated is generally called a message data or information. Any communicated matter, be it audio-visual or otherwise sent by any means is a message or data by itself.

The communication system can be classified into three categories:

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One-way communication 

Here the transmitter only communicates message or information to the receiver, but the receiver, in exchange, cannot communicate anything

Communication System One Way Of Comminication

Example: Books, music, radio, and TV one-way intercom are examples of this kind

Both-way or full-duplex or interactive communication

In this type of communication, the receiver receives information from the sender, and at the same time, it communi¬ cates counter-message or information to the sender. In this way-there goes on a mutual message-transfer transmitted between the sender and the receiver.

Communication System Interactive Comminication

Half-duplex communication

In between these two categories, there is a system where there is a scope of exchanging me$sage£both ways, but which never happens at the same time

Example: SMS or e-mail, through which only after the completion of sending a message from one end, the counter-message can be communicated from the other end. There is no scope for simultaneous message transfer between the sender and the receiver

Timetable of communication system:

Communication System Time Table Of Communication System

In this chapter, we will concern ourselves with distant communication. For communication, within our audible and visible range, no technology is necessary. On the other hand, by applying electromagnetic waves through various technological processes, distant communication has made an incredible development in the last 20/30 years. A list of development of communication science

Elements Of Communication System.

Message signal or data signal:

  • The data or message to be transmitted is converted into a similar electrical wave at the transmitter end. As an audio wave is converted into a similar electrical wave by a microphone.
  • The next step is to transmit this electrical wave through a transmitting antenna, as a similar electromagnetic wave (i.e., as a message signal or data signal) to distant places.
  • In reality, however, the direct transmission of this data signal is practically impossible. Hence, the help of electromagnetic carrier waves is taken.

Carrier wave and transmission band:

Carrier waves are used for carrying the data signal effectively from the transmitter end to the receiver end. The frequency of the carrier wave is much higher than the frequency ofthe data signal i.e., its wavelength is small.

Thus the antennas of transmitters and receivers can function properly. Moreover, the distortion created by the superposition of different data signals at the receiver end is eliminated. Electromagnetic waves of any frequency chosen from a wide range of frequencies can be used as a carrier wave.

But first, the main objective of transmission has to be determined. Then the frequency of the carrier wave is chosen accordingly. For example, for radio transmission up to 200 km, a medium wave (with frequency ≈ 3 × 106 Hz-3 × 106 Hz ) would be adequate. For a particular transmission, the range of frequency used is called the transmission band.

Communication medium or transmission medium:

To reach the receiver end, the carrier wave essentially requires a medium. A medium is selected in such a way that the wave pattern of data signal is received in an undistorted condition.

Generally the media used are:

  1. Atmosphere for wireless communication
  2. Coaxial cable and
  3. O(ptical fiber for wired communication.

Communication channel:

Th transmission medium and the transmission band together form a communication chanel.

‘A short wave is being transmitted through the atmosphere statement indicates a particular communication channel with frequencies between 3 × 106 Hz-3 × 106 Hz. Different zones of atmosphere are chosen for the transmission of different frequencies. The technological means to be adopted for this are also predetermined. Hence, in communication science, the concept of communication channel is of paramount importance

Terminologies used in communication system


As the frequency of the original message or data signal is very low, it cannot travel a very long distance on its own. So, for transmitting it through a long distance, it is super¬ imposed on a high-frequency carrier wÿve. The process of imposing data signal on a carrier wave of specific amplitude and frequency is called modulation. Some important modulations are >0 Amplitude Modulation .. (AM),0 Frequency Modulation (FM), and Phase Modulation (PM).


The modulated wave being transmitted from the transmitter end is collected at the receiver end. Once again the data signal is separated from the wave. This is called demodulation. Arrangements are made to convert the separated data signal 1 again into the original message or data. As, to con¬ vert demodulated audio signal into sound, the loudspeaker is used.


The disturbance or distortion of the original wave¬ form of the transmitted signal is called noise. In practice, however, when a signal is transmitted from the transmitting station to the receiving station, or from some intermediate receiving station to the next receiving station in steps, some noise gets involved with the original signal at every step. As a result, the received data cannot be exactly similar to the transmitted data. As, we can easily differentiate the original voice of the speaker, from the sound emitted from the loudspeaker.

Generally, the noise is classified into two categories:

  1. Controllable noise: Defects in the equipment or their components give rise to this type of ‘noise which can be largely eliminated by using improved instruments. Hence, this defect is most often not treated as noise at all. Obviously, by using high-quality loudspeakers, the degree of noise can be reduced substantially.
  2. Random noise: These noises are not at all within our control. Hence these types of noises can never be eliminated

Block diagram of a communication system

In the main components of a standard communication system is shown in the form of a block diagram

Communication System Black Diagram Of Communication System

Community System Types Of Transmission Data

In distant communication, the transmitted data or messages are basically of three types. Those are described below.

Audio data

Transmissible dialogue or any other sound is audio data. Each piece of information is generated as a sound wave with a frequency between 20 Hz to 20,000 Hz (Audio Frequency or AF). The entire range of this frequency is called the audio frequency range. With a microphone or similar arrangements, these sound waves are converted into similar alternating voltages.

To transmit it to the receiving end, a transmitting antenna is used, which transmits the alternating voltage as electromagnetic waves. The electromagnetic waves should be exactly similar to the original sound, which requires the frequency of the original sound wave and electromagnetic wave to be identical.

The changes in the amplitude of these two waves also have to be identical. This electromagnetic wave Is the audio signal. As the audio message is a sound wave, its speed in the air at 20°C is approximately 330 m. s-1 . But the audio signal is an electromagnetic wave, its speed in air is 3 × 108 m. s-1

Let the frequency of an audio message be 3 kHz. As the frequency of the generated audio signal has to be the same, the wavelength of the signal is

λ = \(\frac{\text { velocity }}{\text { frequency }}\)

= \(\frac{3 \times 10^8 \mathrm{~m} \cdot \mathrm{s}^{-1}}{3 \times 10^3 \mathrm{~s}^{-1}}\)

= 105 m

= 100 km

To transmit this signal directly through a transmitting antenna an antenna of the same size as the wavelength is required, which is practically impossible. For this reason, no audio signal can be transmitted directly from the transmitter end to the receiver end. This constraint applies not only to audio signal but to the following two messages (data) signals also

Audio-Visual or video data

Picture, handwritten or typed matter, still or video photographs, everything in the field of transmission is called video data. In TV and similar transmis¬ sions, audio data is superimposed on video data and these are called audio-visual data. But generally, the term ‘video data’ itself covers audio and visual data combined.

As a microphone is used for converting audio data to audio signal, a camera scanner Is likewise the necessary setup used for converting video data into video signals. This camera can scan a picture by a special process and at the same time, can generate similar alternating electromagnetic video signals conforming to the brightness and color of the picture.

Digital data

Digital data comprises a sequence of lay¬ ers of low and high potentials, where the magnitudes of the potentials are not Important. Bather, the duration of the alternat¬ ing voltages in different layers plays the decisive part In digital transmission, the original audio and visual data are not converted into analogue alternating voltage.

With the advancement of digital electronics, today many types of data, including computer data, are generated as digital data. By applying a special process to this digital data, electromagnetic digital signals are generated. In practice, however, no data signal Is suitable for direct distant communication.

Communication System Digital Data

There are two reasons for this:

  1. From a technical point of view, it needs a huge-sized transmitter and receiver antenna and that is impossible to manufacture.
  2. Voluminous transmitted data from many transmitting cenÿ could crowd the receiving station % at the & same time. Their superposition may generate a fuzzy and unrecognizable signal.

If the data signal contains a single frequency, then the waveform is sinusoidal. But in reality, no waveform of a signal is purely sinusoidal. Hence whatever form it may have, it is treated as a wave generated from the superposition of many sinusoidal waves.

Communication System – Carrier Wave

The carrier wave is used to remove the above two difficulties in distant communication of data signals. The wave is so named because the wave carries the data signal from the transmitter end to the receiver end. The main feature of the carrier waves is that their frequency is many times higher than the frequency of any data signal.

In other words, its wavelength is much smaller than that of data signals. In this connection, it should be remembered that the range of frequency of the audio signal as data signal is from 20 Hz to 20 kHz, and this range is called the Audio Frequency range or AF range.

Each carrier wave is an electromagnetic wave. Normally, waves of lower frequencies from the electromagnetic spectrum are used as carrier waves. Generally, these electromagnetic waves are radio waves and microwaves frequency range of 3 kHz to 300 GHz. In the field of communication, the use of the residual part of the spectrum is almost non-existent. Considering the different objectives and usages in distant communication, the carrier waves are classified into several categories.

Communication System Carrier Wave

When a data signal is to be transmitted from one place to another, the suitable carrier wave is selected at the very beginning. From the table, it is clear that the frequency which is suitable for radio transmission is not suitable for TV transmission


An ideal carrier wave is a purely sinusoidal electromagnetic wave with a definite frequency. However, any attempt to generate a wave invariably results in the main frequency being superimposed with several other higher or lower frequencies. The intensity of the carrier wave is maximum for only its main frequency f0. However, the intensities of other frequencies on either side of f0 diminish gradually

Communication System Band Width

Suppose that the intensity of each of the frequencies f1 and f2 on either side of f is half of the maximum intensity, which means that the rate of energy or power carried is also equal to half of the maximum value. These two frequencies a f1nd f2 are called half-power frequency.

Only when the frequency gets lower than f1 or higher than f2, the intensity of the wave becomes negligible. So, though the main frequency of the carrier wave is f0, the intensity or amount of energy carried by the waves between the frequencies f1 and f2 cannot be ignored. This difference of frequencies (f2-,f1) is called the bandwidth of the carrier wave. As data signal is superimposed on carrier waves, it is also called signal bandwidth.

As a frequency-intensity graph becomes sharper, the bandwidth gets reduced. Hence the problem due to the mixing of different carrier waves is also reduced. As per international norms for radio transmission, it is desirable that the band¬ width should not exceed 5 Hz, and the difference between two main frequencies (JQ) of carrier waves, transmitted from two transmitting stations, should be at least 10 kHz.

Each carrier wave of every transmitting station carries its own data signal. The carrier waves are recognized at the receiving stations, only due to the difference of their frequencies. Hence the data signals do not become fuzzy or unrecognisable by intermixing i.e., remain distinct from each other.

Communication System – Modulation And Demodulation

It has been mentioned that the process of superimposing the low-frequency data signal on a high-frequency carrier wave at the transmitting end is called modulation. On the other hand, the process of separating the superimposed data signal from the carrier wave at the receiving end is called demodulation.

Let the equation of carrier wave be,

V= V0 sin(Ωt + θ)

[Where, V0 = amplitude of the wave, Ω = Angular frequency, Ωt + θ = phase of the wave = Φ(say), θ = initial phase or epoch

Linear frequency or simply the wave frequency

n =  \(\frac{\Omega}{2 \pi}, \text { where } \Omega=\frac{d \phi}{d t}\)

For convenience, the initial phase 6 can be taken as zero.

In that case,

The carrier wave: V = V0 sin Ω t ……………………………………… (1)

Similarly, the equation of the data signal,

v0 = v0  sinωt  ………………………………………… (2)

[where, v0 = amplitude of the wave and co = angular frequency]

The instantaneous voltage v is called the modulating voltage

The condition of effective modulation of the carrier wave of equation (1) by modulating the data signal of equation (2) is,


The frequency of the carrier wave should be much higher than the frequency of the data signal. In practice, this frequency is 1000 times or even more than that of the data signal. The modulating voltage generally fluctuates very slowly with time and hence the wave indicating data signal is frequently called slow wave.

Amplitude Modulation (Am) And Amplitude Demodulation

Amplitude Modulation or AM Definition:

In the case of distant communication, after the imposition of a data signal on a carrier wave, if the frequency remains unchanged and only the amplitude changes in a slow, periodic way, then the process is called Amplitude Modulation or AM.

Let, on the carrier wave of equation (1)  the data signal of equation (2) be superimposed in such a way, that the value of amplitude V0 changes with time. With the stationary value of amplitude V0 of the carrier wave, a sinusoidal alternating voltage kv0 sincot gets added, i.e.,

Vm0(t) = V0 +kv0sinωt ……………………… (1)

Where k is a dimensionless constant. Vm0 is the amplitude of generated modulated voltage due to modulation of a carrier wave by data signal.

As ω<<Ω, so the change of voltage is very slow.

Modulated voltage, Vm = Vm0 sinΩt

= (V0 +kv0 sinωt )sin Ωt

∴ V= V0 (1+β+sin ωt ) ……………………..(2)

Where β = \(k \frac{v_0}{V_0}[/atex] = modulation index

The significance of equation (2):

  1. The existence of sinΩt indicates that, due to modulation, the frequency of carrier wave (Ω) does not change with time.
  2. sin ωt can be both negative and positive. As the value of co is very low, the multiplier (1 + βsinωt) of the amplitude V0 indicates that the amplitude of the modulated wave fluctuates very slowly on either side of V0.

The carrier wave is shown as a sinusoidal curve of a particular frequency and also a data signal is shown as a sinusoidal curve of another particular frequency. In both cases, with the two waves, their corresponding alternating sine voltages are taken as the vertical axis

Communication System Alternating Sine Voltages As The Vertical Axis

Communication System Alternating Sine Voltages As The Vertical Axis.

A carrier wave, when modulated by a data signal, is the nature of the AM wave thus produced.

Communication System Modulated By A Data Signal


From equation (2),

V = V0 sin Ωt +βV0 sinΩt sin ωt

= V0 sin Ωt + [latex]\frac{\beta V_0}{2}\) cos (Ω – ω)t – \(\frac{\beta V_0}{2}\) cos (Ω + ω)t ……………… (3)

Equation (3) reveals that AM wave consists of sinusoidal components of three frequencies Ω, Ω- ω, and Ω+ ω. Though the frequency of the carrier wave does not change with time, the other two frequencies overlap with the main frequency f (both cosine and sine functions are sinusoidal).

In this discussion, the data signal has been taken as composed of a single frequency co. In audio signal, can be considered to be a signal generated from a pure tone, and in video signal, is a monochromatic signal. In most practical cases, however, quite a of sinusoidal frequencies lie superimposed on the original data signal.

Let us suppose, in a data signal, frequencies of range ω to ω ±Δω are mixed up. In that case, if the (ω → ω ±Δω) range is considered in place of co in equation (3), then the frequencies present in the amplitude-modulated wave are

Ω: Point O

The frequencies of (Ω + ω) to (fl + (Ω + ω +Δω ) range:

Part BD

The frequencies of(Ω – ω – Δω) to (Ω – ω ) range:

Part CA

Communication System Sideband Of Range

Here, the frequency range CA and BD are called the sidebands of the amplitude-modulated wave. The maximum and minimum frequencies of the full wave are (Ω + ω +Δω) (point D) and (Ω – ω -Δω) (point C). The CD stretch of the frequency is called the bandwidth of the AM wave. The magnitude of the bandwidth is,

d = CD = (Ω + ω +Δω)- (Ω – ω -Δω) = 2(ω + Δω)

= 2 × maximum frequency on the data signal

To transmit the AM wave properly, a communication channel is to be selected, the transmission band of which should be extended at least from (Ω + ω +Δω) to (Ω – ω -Δω) i.e., from point C to D . However, instead of both the sidebands, transmission by using only one sideband is also in practice. In that case, transmission can be done with half bandwidth. This is called single-sideband communication.

Demodulotion principle of AM wave

A well-selected circuit performs the function of amplitude modulation. Now the modulated wave is applied to a transmitting antenna. The waveform of the electromagnetic wave thus generated and spread all over is exactly similar to the modulated waveform.

Transmitted through the communication medium, the electromagnetic wave reaches the receiving antenna (one or more than one), and in each antenna, an exactly similar potential wave is generated from the electromagnetic wave. Naturally, the intensity of the wave at the receiving end is much less than that at the transmitting end.

In the next step, keeping the waveform intact, the intensity ofthe wave is multiplied several times by using a suitable amplifier. Then, with suitably selected circuitry, the carrier wave and the data signal are separated from the mixed wave. This is the process of demodulation its other popular name is detection. Demodulation of the amplitude-modulated wave is also called Peak Detection.

Radio receiver:

At last, the demodulated data signal is amplified again, and with the right procedure(as in audio signal, through loudspeaker) the data generated from the transmitting end, is traced. In audio signal, the entire machinery set up is called a radio receiver.

Communication System Radio Receiver

It can be mentioned here that, the process of demodulation of frequency-modulated wave Is also exactly similar to this though the circuitry Is different.

Production of an AM wave

An n-p-n transistor T. At the very beginning, a dc circuit of the transistor Is constructed by using the necessary bias voltage and different resistances. An operating point Is also fixed. These dc components of the circuit have not been

Communication System Production Of An AM Wave

Now by using the appropriate DC filter, the alternating voltage of the carrier wave is applied to base B of the transistor, and the alternating voltage of the data signal is applied to collector C. Depending on the fluctuation of the voltage of the data signal, the collector current changes and thus, at output of transistor, an identical replica of data signal gets mixed on the amplified carrier wave and an amplitude modulated wave is generated. Necessary filter arrangements are made so that no DC component of the current can mix with it.

Amplitude modulators are basically of two types:

  1. Linear modulator, used in radio transmission
  2. Square-law modulator, used in a telephone conversation

Defection of an AM wave

The circuit is widely used as a demodulator of AM waves. It is called an envelope detector. It is a half-wave rectifier, specially made up of a semiconductor diode D, in the circuit to which an RC filter is connected.

When the voltage reaches its highest value in the positive half cycle of the carrier wave of the AM wave, the charge of capacitor C becomes maximum. Now, as the carrier voltage gradually decreases, the charge of C starts to reduce due to dissipation through R.

The time constant of RC circuit Le., CR is kept at such a magnitude that, it becomes much more than the time of the carrier wave and a little less than the period of data signal. As a result, the charge of C does not Increase or decrease as fast as that ofthe phases of a carrier wave. Rather, the waveform of the data signal is manifested through the increase or decrease of the charge of C.

At output, the waveform of the carrier is not available, the output wave mainly indicates the waveform of the data signal. This envelope detector is a linear detector, because, here the output voltage becomes proportional to the input voltage. To ensure that no component of DC can pass to output, capacitor C’ is used as a filter in the circuit, as

Communication System Detection Of An AM Wave

Frequency Modulation And frequency Demodulation

Frequency Modulation Definition:

In distant communication, after superimposing a data signal on a carrier wave, if the amplitude remains unchanged and only the frequency changes in a slow periodic way, then the phenomenon is called Frequency Modulation or FM.

Equation of carrier wave,

V = V0sinΩt = V sin Φ ………………………………….. (1)

Where phase angle, Φ = Ωt

i.e  \(\frac{d \phi}{d t}=\Omega\)  = Ω …………………………………. (2)

And, The equation of data signal

v = v0 sincωt …………………………………. (3)

Keeping the amplitude V0 of the carrier wave unchanged, the time-dependent data signal is superimposed on it.

Due to modulation, the instantaneous value of frequency Ω is,

Ωm (t) = Ω + kv0 sinωt …………………………………. (4)

Here, k is a coupling constant that converts v0 sinωt into a frequency-dependent term.

In this case, the unit of k will be V-1. s-1

( kv0 sinωt) will be s-1 , i.e., Hz. Applying equation (2) for this time-dependent frequency we get,

m =  Ωm (t)dt

Or, Φm = ∫ Ωm (t)dt= Ω ∫ dt + kv0 ∫ sinωt dt

Or, Φm = Ωt – \(\frac{k v_0}{\omega} \cos \omega t+\delta\) cosωt + δ …………………………. (5)

Writing time-dependent phase angle Φm in place of Φ in equation (1) of carrier wave we get

Vm = V sin ( Ωt – β cosωt + δ) …………………………… (6)

Where,  β = \(\frac{k v_0}{\omega}\)

This is the equation of frequency modulated wave or FM wave. In this equation

β = \(\frac{k v_0}{\omega}\)

In this case:

The amplitude V0 of the carrier wave does not change.

For the term (β cosωt), the frequency of carrier wave fluctuates with time.

As ω<<Ω, this change of frequency is slow. This phenomenon is called frequency modulation.

A carrier wave, a data signal, and a form of FM wave, generated due to the superimposition of these two waves, are shown respectively.

Communication System Frequency Modulation And Frequency Demodulation

It is observed that in the AM wave, besides the frequency Ω of the carrier wave, two more sine components of frequencies (Ω – ω) and (Ω + ω) are present. But on mathematical analysis of frequency modulated wave, it is found that the number of sine components is infinite. The frequency of the components are Ω ±ω, Ω ±2ω, Ω ±3ω, ………………. etc.

These frequencies lie symmetrically on either side of the frequency of the carrier wave Ω.

In transmission through Low Frequency (LF) or Medium Frequency (MF) carrier wave, amplitude modulation is generally employed while in transmission through High Frequency (HF) or Very High Frequency (VHF) carrier wave, frequency modula¬ tion is employed. The range of frequencies of HF or VHF is very high, so the differences in frequency of the carrier waves, transmitted from different transmitting centers, are also very high.

Hence, despite the presence of sideband frequencies in FM waves, the possibility of overlapping of different waves and noise and distortion of data signal due to this becomes very low. For this reason, the sound of FM radio is very clear and in TV, the chance of mixing pictures transmitted from two stations is almost nil.

Demodulation principle of FM wave

In most cases, the receiver generates another waveform the frequency-modulated wave, and the slow change of amplitude is associated with the slow change of frequency. The wave is then applied to a suitable detector circuit and almost in the same method as in the demodulation of amplitude-modulated wave the data signal is separated.

Nowadays in the demodulation of FM waves, the use of a superheterodyne receiver is prevalent. Here an oscillator is kept inside the receiver from which a wave of properly selected frequency is mixed with the FM wave. The process is very useful for detecting the correct frequency of data signal

Power Dissipated due to Modulation

If the potential difference across the two ends of a transmitting antenna is time-dependent and the potential difference is expressed as V(t), then the dissipated power of the antenna is

P = \(\frac{\overline{V^2(t)}}{R}\) ………………………….(1)

Here, the symbol \(\overline{V^2(t)}\) indicates the mean of the quantity Vt concerning time. R is the effective resistance of the antenna. In the case of of time-dependent voltage, due to the metallic elements of the antenna and electromagnetic radiation from it, the effective resistance R is not equal to the DC resistance of the antenna

Now, according to equation (1)

Carrier wave: V = V0 sin Ωt …………………………. (2)

Again, from equation (2), we get,

AM wave: VAM = V0(1 + β sinωt)sinΩt ………………………….. (3)

Similarly from equation (6) we get
FM wave: VFM = V0sin(Ωt – β cosωt) ……………………………. (4)

In both equations (3) and (4), the term is modulation index which is sometimes expressed by the symbol ‘m’ also. Each of V, VAM, and VFM indicated by the above three equations is time time-dependent potential difference. Hence, in each case, the power dissipated in the antenna is to be evaluated by using equation (1)

The power dissipated In the transmission of carrier wave:

From equation (’2)

V2 = V02 sin2 Ωt

= \(\overline{V^2}=V_0^2 \overline{\sin ^2 \Omega t}\)

We know that functions like sin Ωt If or cos Ωt are time-dependent sinusoidal functions, lit each complete cycle, the average of sin²-function or cos²-function is zero, but the average ol sin or cos-function is ½.

∴ \(\overline{\sin ^2 \Omega t}\)  = ½

Hence, the power dissipated hr antenna for transmitting carrier wave only:

PC = \(\frac{\overline{V^2}}{R}=\frac{V_0^2 \cdot \frac{1}{2}}{R}=\frac{V_0^2}{2 R}\)  ……………………………… (5)

The power dissipated In the transmission of amplitude-modulated wave:

From equation (3) we get,

AM =  V²0( 1+β sinωt)² sin²Ωt

= V²0( 1+2β sinωt +β² sin² ωt)  sin²Ωt

So,  \(\overline{V_{A M}^2}=V_0^2\left(1+2 \beta \overline{\sin \omega t}+\beta^2 \overline{\sin ^2 \omega t}\right) \overline{\sin ^2 \Omega t}\)

= (1+ 2β.0+β .½). .½ = \(\frac{V_0^2}{2}\left(1+\frac{\beta^2}{2}\right)\)

Hence power dissipated in the antenna,

PAM =  \(\overline{\frac{V_{A M}^2}{R}}=\frac{V_0^2}{2 R}\left(1+\frac{\beta^2}{2}\right)\)

Or, PAM =  \(P_C\left(1+\frac{\beta^2}{2}\right)\) ……………………………………… (6)

The significance of this equation is that the power dissipated in the antenna increases in transmitting amplitude modulated wave compared to that in transmitting only the carrier wave. The amount of increase is

P’= PAM – PC = \(P_C\left(1+\frac{\beta^2}{2}\right)-P_C=P_C \cdot \frac{\beta^2}{2}\)  ……………………………… (7)

i.e., the rate of dissipation of additional power or energy is proportional to the square of the modulation index. As, for 100% modulation, β = 100% = 1

So, PAM =   \(P_C\left(1+\frac{1^2}{2}\right)\) = \(\frac{3}{2}\) PC

In this case Increase In dissipated power

P’ = \(\frac{3}{2}\) PC  – PC = \(\frac{1}{2}\) PC

i. e amount of Increase Is 50%.

It may the noted that dial tho additional dissipated power duo to amplitude modulation Is equally divided between die two side hands. I,e„ dissipated power for each sideband

= \(\frac{p^{\prime}}{2}\)

= PC. \(\cdot \frac{\beta^2}{4}\)

The power dissipated In the transmission of frequency-modulated wave:

Prom equation (4),

⇒ \(v_{l M}^2=v_0^2 \sin ^2(\Omega-\beta \cos \omega t)=V_0^2 \sin ^2 \phi_{l_M}\)

Where ΦFM = Ωt – β cosωt

So, \(\overline{v_{P M}^2}=v_0^2 \overline{\sin ^2 \phi_{I M}}=v_0^2 \cdot \frac{1}{2}\)‘

Hence, energy dissipated in the antenna

PFM = \(\frac{\overline{V_{I M}^2}}{R}=\frac{V_0^2}{2 R}\)  = PC ……………………….. (8)

The significance of this equation Is that, the power dissipated in the antenna remains unchanged In transmitting frequency-modulated waves compared to that In transmitting only the carrier wave. That is, for frequency modulation, there is no change in dissipated power. This power is not at all dependent on the modulation Index, β.

Modulator-Demodulator or Modem

It is a device that converts digital data to analog data and analog data to digital data. Modern computers function on a digital scale. All the data available from it is digital. In a localized area (as in an office), where the local area network or LAN is formed by connecting the computers of the office, direct exchange of digital data takes place. In such cases, the data need not be converted to analog data, hence there is no need to use the modem. In this type of direct exchange of digital data, mainly coaxial cable is used as the medium.

On the other hand, in distant communication, the computer is connected to a telephone line through a modem. The digital data of the computer is converted into amplitude-modulated or frequency-modulated analog data by a modem. Then this analogue data reaches the computers of the worldwide network through the telephone system. The modern connection with the receiving computer on its part separates the primary digital from the modulated analog data.

Communication System – Propagation Of Carriers Waves Through Atmosphere

When electromagnetic carrier waves of range from Very Low Frequency (VLF) to microwave (as stated in table 1) propagate through the atmosphere.

Mainly three types of waves are produced

  1. Ground wave
  2. Space wave
  3. Skywave

Suitable waves for transmission and its corresponding range of frequency of carrier wave are to be selected first and their technological set-ups are to be arranged accordingly.

1. Ground wave: This wave propagates from one point to another, along the surface of the earth following the earth’s curvature. For the propagation of this wave, the earth’s surface at one side and the lowest layer of the ionosphere (which is nearly 80 km above the earth’s surface) at the other both act as reflectors. The wave progresses through successive reflections which it suffers from these two surfaces.

Communication System Ground Wave Transmitting Antenna

Another special feature of the wave is the polarisation of the transmitting antenna is placed vertically, so the wave gets polarised at a vertical plane at the start i.e., its electrical field has no horizontal component and initial wavefront is a vertical wavefront.

Now, as the wave progresses along the earth’s surface, the wavefront gradually gets tilted from the vertical line, which means that the magnitude of the horizontal component of its electrical field starts Increasing. The horizontal component of the transmitted energy gets absorbed on the earth’s surface. As the tilt of the wavefront towards the horizontal plane increases, the rate of absorption of energy continues to Increase. Therefore ground wave is not suited for long-range communication.

Communication System Tilting Of Groung Wave With The Time

Ground wave plays a very important role in transmission through carrier waves of Very low Frequency (VLF) and Low Frequency (LF). In the case of higher frequencies, the energy carried by ground waves gets absorbed very fast. This is why FM radio or television signals can be transmitted only up to a short distance.

2. Space wave:

This type of wave can propagate through the atmosphere in a straight fine almost without any deviation. Due to the curvature of the earth’s surface, we can communicate through rectilinear space wave up to a distance of a certain limit. It Is called the radio horizon of the corresponding antenna

The space wave can propagate up to a maximum point O from the transmitting antenna. The distance PO of the earth’s surface is the radio horizon of the antenna.

Communication System Space Wave

Let R = radius of earth

H = Height of transmitting antenna A

From we get

cosθ = \(\frac{O C}{A C}=\frac{R}{R+H}=\frac{1}{1+\frac{H}{R}}\)

= \(\left(1+\frac{H}{R}\right)^{-1}\)

cos²θ = \(\left(1+\frac{H}{R}\right)^{-2}\) ≈ 1 – \(\frac{2 H}{R}\)

(As H<<R)

Now sin²θ  = \(\frac{2 H}{R}\)

Or, sin= \(=\sqrt{\frac{2 H}{R}}\)

As θ is very small, sin θ ≈ θ

So,  θ = \(=\sqrt{\frac{2 H}{R}}\)

Hence, radio small, horizon,

PO = Rθ

= R \(\sqrt{\frac{2 H}{R}}\)

= \(\sqrt{2 H R}\) ……………………………….. (1)

Similarly in the radio horizon of receiving antenna B, QO= \(\sqrt{2 R h}\)

When both transmitting and receiving antennas are used at the same time, the magnitude of the radio horizon is,

PQ = PO+QO = \(\sqrt{2 H R}+\sqrt{2 h R}\)

= \(\sqrt{2 R}(\sqrt{H}+\sqrt{h})\)  ………………………………………….. (2)

For example, if a space wave, transmitted from a transmitting antenna, 300 m high, is to be received by a receiving antenna 30 m high, the radio horizon will be.

= \(\sqrt{2 \times(6400 \times 1000)}(\sqrt{300}+\sqrt{30})\)

≈  81.6 ×10³ m = 81.6 km

[The radius of the earth has been taken as 6400 km ]

If carrier wave of High Frequency (HF) or of even higher frequency Is transmitted as a space wave, then a little energy is absorbed. So, despite the limitations of radio horizon, space waves are most suitable for very long-distance transmission.

In this process, the application of one or more Intermediate is most prevalent. Each intermediate antenna receives the transmitted signal and transmits the signal to the next antenna. In this way, the radio horizon can be enlarged to a great extent.

As a high-quality communication technique, wave communication Is Increasing day by day. The frequencies 8 of these microwaves arc very high, 300 MHz to 300 GHz.

These waves are transmitted mainly as space waves. For long-distance communication, instead of using a series of antennas, artificial earth satellites are employed. Space waves transmitted from earth surface after getting reflected from the artificial satellites, Let R = radius of the earth, reach distant places on the earth’s surface. This arrangement is called satellite communication


Carrier waves of Medium Frequency (MF) or -I Lower High Frequency (LHF) are mostly transmitted as sky waves. In sky wave propagation, the ionosphere extending from a height of BO km to about 400 km above earth’s surface plays a very important MLC. The ionization of air in the ionosphere Is mainly due to solar radiation in the daytime and cosmic rays of much less intensity during the night.

Hence the rate of ionisation Is much greater during the daytime than at night. In the ionosphere, electrons and ions formed due to ionization remain mixed with neutral molecules anti atoms. The rate of ionization is measured by the number of free electrons In unit volume, l.e., by electron number density.

Her two factors are Important:

  1. Since solar radiation or cosmic radiation falls on the upper surface of the ionosphere first, the rate of ionization is higher in the upper surface. Depending on the difference of rate of ionization, the ionosphere Is divided into three lay upper layer, middle layer, and lower layer. The rate of ionization is maximum at the upper layer. The middle and the lower layer almost do not exist at night.
  2. With the change of Ionisation rate in air, the velocity of two magnetic waves through it also changes. As a result, the ionosphere acts as a refractive medium. I higher the ion-isolation rate in a layer, higher is the velocity of electromagnetic waves through It. That Is, the refractive index of this layer decreases. In the layers of the Ionosphere, , the rate of ionization of a higher layer A  Is maximum and Its refractive index is minimum. It can be proved that for. an incident electromagnetic wave of frequency f, the refractive index of a layer of Ionosphere

Communication System Velocity Of Electromegnetic Waves

μ = \(\left(1-\frac{81 n}{f^2}\right)^{1 / 2}\)…………………………. (3)

Where, n = electron number density of that layer of the medium. Whenever a sky wave propagates through the ionosphere, to a higher layer, it advances from a higher refractive index layer to a lower refractive index layer.

When the slope of an incident sky wave concerning the vertical axis becomes very small, it is refracted from the highest ionization layer i.e., from the lowest refractive index layer, and escapes the ionosphere (ray ‘a’ ) On the other hand, if any wave with a higher slope is incident from below onto the ionosphere, it suffers total internal reflection from one or the other layer and is reflected to the earth surface.

In other words, it is due to reflection in the ionosphere that the sky wave is transmitted from one point of the earth’s surface to another. As the values of f are relatively low for the waves of Medium Frequency (MF) the refractive indices of the layers of the ionosphere, by equation (3), are also low. As a result, from even the lower layer of the medium, the sky wave gets reflected. That is why sky waves of such frequencies travel relatively shorter distances on Earth surface

On the other hand, sky waves of High Frequency (HF) (i.e., short wave) are reflected from the upper layers of the ionosphere. Hence, the reflected waves can reach distant places on earth’s surface. In this case, these waves are rather absorbed to some amount in medium and lower layers, hence the intensity of the wave is reduced.

Again at night, as the middle layer and lower layer almost do not exist, the amount of absorption is negligibly small. Hence, the signal carried by sky waves is more intense and clear at night. Skywave plays the main role in radio transmission. Normally, medium wave transmission for short distances and short wave transmission for longer distances are made through sky waves.

Other media for the propagation of earner waves

It has already been mentioned that apart from air medium, two other media are used for communication. These are:

  1. Coaxial cable: For short-distance communication, this medium is most effective.. As, in a telephone communication system or a computer network limited to a ‘ small area, the coaxial cable is widely used.
  2. Optical fiber: Unimaginable success has been achieved in worldwide telephone communication systems by using optical fiber. Through optical fiber, it is possible to transmit electromagnetic waves in the range of almost infinite frequency, very effectively.

[The detailed discussion regarding these media is not included in the present syllabus.]

Communication System  – Communication Channel

Two factors are to be ascertained first to decide how to transmit a data signal from one place to another.

  1. Medium: Atmosphere, coaxial cable, or optical fiber are used as a medium.  Sometimes combination of different types of media is also used.
  2. Range of frequency: What part of the electromagnetic spectrum would be used as a carrier wave, is also to be trained. The corresponding range of the frequency is called the transmission band.

The arrangement for transmission of data signal from one point to another with a combination of an appropriate medium and a carrier wave of appropriate frequency is called a communication channel.

As, the carrier waves, of frequency in the range of approximately 300 kHz to 10 MHz, can propagate as sky waves through the atmosphere from one place to another. Hence it is an effective communication channel. The efficiency of a communication channel is determined by the total number| of discrete data signals that can be transmitted through it, without distortion.

As, a carrier wave of frequency 1 MHz carries an audio signal Naturally, the frequency of the audio signal lies between 20 Hz to 20000 Hz. If the carrier wave is amplitude-modulated by the audio signal, then the bandwidth ofthe amplitude modulated wave would be =2 × 20000 = 40000 Hz. This band lies symmetrically on either side of the main frequency and its location is between (106-20000) Hz or 930000 Hz to (106 + 20000) or 1020000 Hz. Now if an audio signal is transmitted from another transmitting station through a carrier wave of frequency 1.05 MHz, the location of the band would be between the frequency 1030000 Hz to 1070000 Hz.

There is a distinct difference between the former band and the present band and they will not mix up together, so the receiver can clearly distinguish the two audio signals. It is clear from this example that, since the bandwidth of the audio signal is 40 kHz hence, if the difference of frequencies between two carrier weaves be more than 40 kHz, the audio signals can be received distinctly by the receiving machine.

Now we can consider the propagation of skywave through atmo¬sphere. Here the propagation of effective frequency is from 100 kHz to 10 MHz, Le., the width of the transmission band is 107-105 ≈ 107 Hz = 10 MHz. Now, for a clear and distinct trans¬ mission, the difference of frequency between the carrier waves should be at least 0.04 MHz. For example, if this difference is taken as 0.05 MHz, then through the 10 MHz width of the transmission band, (10 ÷ 0.05), 200 carrier waves can pass together, which means 200 audio signals can be transmitted distinctly through the sky wave band.

In reality, since frequencies higher than 4000 Hz are almost absent in audio signals, the bandwidth of amplitude-modulated waves is very narrow. Hence, it is sufficient if the difference of frequencies between the carrier waves is 0.01 MHz (or 10 kHz). In that case, more audio signals can be transmitted by the sky wave communication channel through the atmosphere.

In the context of the modern progress of telecommunication and internet, the use of some super-efficient communication channels has become essential. Microwave communication, use of artificial satellites, application of optical fiber as a medium all these steps develop a channel communication system of infinite range, through which an extremely large number of data signals can pass. This is called a broadband communication system

Communication System Communication Channel Numerical Examples

Example 1. U if the height of a television tower is 300 m would the TV transmission be possible? (Given, the radius of earth = 6400 km) , how far

Given the height of the tower, h = 300 m

Radius of earth, R = 6400 km = 6.4 × 106 m

The required distance,

d = \(\sqrt{2 R h}\)

= \(=\sqrt{2 \times 6.4 \times 10^6 \times 300}\)≈ 6.4 ×104 m

= 62 km

Example 2. At what height is the transmitting antenna to be placed to make a TV transmission up to a distance of 32 km? The radius of the earth = 6.4 × 106 m.

Given the radio horizon,

d = 32 km = 32 × 106 m

Let the transmitting antenna be placed at a height h

We know, d = \(\sqrt{2 R h}\)

Or, h = \(\frac{d^2}{2 R}=\frac{\left(32 \times 10^3\right)^2}{2 \times 6.4 \times 10^6}\)

= 80 cm

Example 3. Equation of a wave: V = 10sin(106 t + 0.4sin 1000t). What is its index of modulation? or, 80 m

The given wave is frequency-modulated. General equation of the wave, V0 = V0 sin(Ωt + βsin ωt) Comparing this with the given equation we get, modulation index, β = 0.4

Example 4.  Electron number density in a layer of ionosphere is 4 × 105 cm-3. For an electromagnetic wave of frequency 40 MHz, what would be the refractive index of ; that layer?

Frequency f = 40 MHz = 40 × 106 Hz

n = 4 × 105cm-3 

=  4 × 105 × 106 m-3  =  4× 1011 m-3

∴ Refractive index

π = \(\left(1-\frac{81 n}{f^2}\right)^{1 / 2}=\left[1-\frac{81 \times 4 \times 10^{11}}{\left(40 \times 10^6\right)^2}\right]^{1 / 2}\)

= (1- 0.02025)½

= \(\sqrt{0.97975}\)

= 0.99

Example 5.  Modulation Index of an amplitude modulated wave β = 50% and power dissipated In transmission 18 kW. What is the rate of energy dissipation for each sideband?

Modulation index, β = 50% = \(\frac{1}{2}\)

We, know PAM = PC \(\left(1+\frac{\beta^2}{2}\right)\)

Or, 18 = PC \(\left[1+\frac{\left(\frac{1}{2}\right)^2}{2}\right]\) = \(\frac{9}{8}\) PC

i.e. Power of carrier wave,

PC = 18 × \(\frac{18}{9}\)

= 16 kW

Increase in power , due to modulation = 18- 16 = 2kW

This power is distributed equally in the two sidebands

Rate of energy dissipation for each sideband

= \(\frac{2}{2}\)

= 1 Kw

Unit 10 Communication System Synopsis


Communication System Elements Of Communication System

1. At first, the message or data to be communicated, is to be converted into a similar electrical wave at the transmitting end.

2. The electrical wave is transmitted by a transmitting antenna as a similar electromagnetic wave, i.e., a data signal, to distant places.

Communication System Transmitted Data

4. Electromagnetic carrier wave is used to carry the data signal from the transmitting end to the receiving end. The frequency of the carrier wave is much higher than the data signal.

5. For a particular transmission, the range of frequency used as carrier wave is called the transmission band

6. As per internationally accepted rule, it is desirable that for radio transmission the bandwidth should not exceed 5 kHz. Based on it, the difference between the two main frequencies of the carrier waves transmitted from two transmitting stations should be at least 10 kHz.

7. The process of superimposing data signal on carrier wave at the transmitting end is called modulation. On the other hand, the process of separating the imposed data signal from the carrier wave at the receiving end is called demodulation.

8. In long-distance communication, after the superimposition of a data signal on a carrier wave, if the frequency remains unchanged and only the amplitude changes slowly and periodically, then the process is called Amplitude Modulation (or AM in brief).

10. In long-distance communication, after the superimposition of a data signal on a carrier wave, if the amplitude remains unchanged and only frequency changes slowly and periodically, then the process is called Frequency Modulation (or FM in brief).

11. Generally, amplitude modulation is employed in transmis¬ sion through carrier waves of I-ow Frequency (LP) or Medium Frequency (MF). On the other hand, frequency modulation is employed In transmission through carrier waves of High Frequency (HF) or very High Frequency (VHP).

12. Due to amplitude modulation, additional dissipated power is proportional to the square of the modulation Index. 4 Due to frequency modulation, there is no change In dissi¬ pated power. The power does not depend on the modulation index at all.

13. A modem is a device that converts digital data to analog data and analog data to digital one.

Communication System Carrier Waves Through Atmosphere

14. Ground wave advances along the earth’s surface, through successive reflections between the earth’s surface and the lowest layer of the Ionosphere.

15. Ground wave lakes have an effective role In the communication of carrier waves of very to frequency (VLF) or Low Frequency (LP). f or frequencies more than that, (he carried energy by ground wave gets absorbed at a very rapid rate. It is the reason, why PM radio or TV signals can be transmitted only up to a short distance.

16. Due to the curvature of the earth, transmission through straight-line space waves has a limit. The limit of the concerned antenna is called the radio horizon.

17. Using successive antennas, the radio horizon can be enlarged effectively to a great extent.

18. In long-distance communication, in place of sales of antennas, a satellite communication system Is employed.

19. Carrier waves of Medium Frequency (MF) and Lower High Frequency (LHF) are transmitted mainly as sky waves.

20. The wavelength of a wave in a vacuum or In air-medium

λ = \(\frac{c}{f}\)

Where, c = velocity of the water

f = Frequency of the wave

21. Bandwidth of amplitude modulated wave (AM wave) :

d = 2 × highest frequency present In dam signal.

22. Equation of carrier wave: V = V0 sin (Ωt + θ)

And, the equation of dam signal, v = v0sinωt

Where, V0 = Amplitude of the wave,

Ω= Angular frequency of Ute wave,

ω = Frequency of modulating voltage,

v0 = Amplitude of modulating voltage.

23. Equation of amplitude modulated wave,

VAM = V0 (1+β sinωt) sinΩt

Here, β = k \(k \frac{v_0}{v_0}\) = Modulation index

Equation of frequency modulated wave,

VFM = V0 sin(Ωt – β cosωt)

Here, β = \(k \frac{v_0}{\omega}\) = modulation index

24. Dissipated power in the transmission of the carrier wave,

P1 = \(\frac{V_0^2}{2 R}\)

(where R It the effective resistance of the antenna)’

Dissipated power in the transmission of amplitude-modulated wave,

PAM = PC \(\left(1+\frac{\beta^2}{2}\right)\)

Dissipated power for each sideband PC = \(P_C \cdot \frac{\beta^2}{4}\)

25. Dissipated power in the transmission of the frequency-modulated wave. PFM =  PC.

26. Radio horizon = \(\sqrt{2 R h}\)

Where, R = radius of the earth, H = height of transmitting antenna.

Refractive Index of a layer in the Ionosphere, for an electromagnetic wave of frequency f,

μ  = \(\left(1-\frac{81 n}{f^2}\right)^{1 / 2}\)

Where, n = electron number density in that layer of the medium

Community System Very Short Question And Answers

Question 1. What quantity remains unchanged in the FM wave?
Answer: Amplitude of wave

Question 2. What quantity remains unchanged in the AM wave?
Answer: Frequency of wave

Question 3. What is the name ofthe device which converts analog data to digital data and vice versa
Answer: Modem

Question 4. How does the dissipated power change, if a transmitted carrier wave is frequency modulated?
Answer: Remains the same]

Question 5. Which mode of propagation is used by short-wave road cast services?
Answer: Sky wave propagation is used by short-wave broadcast services

Question 6. How does the refractive index of a layer of ionosphere change for the propagation of a radio wave, if the rate of one-
Answer: Refractive index decreases

Question 7. In transmission through the layers of the ionosphere, if the frequency of a radio wave increases, how does the refractive
Answer: Increases

Question 8. Which one between space waves and sky waves, used in distant communication through the atmosphere, has a higher frequency?
Answer: Space Waves

Community System Fill In The Blanks

Question 1. Concerning any data signal, the frequency of a carrier wave is much _________________
Answer: Higher

Question 2. Data signal, communicated through internet is a _________________signal
Answer: Digital

Question 3. The process of superimposing any data signal on a carrier wave is called _________________
Answer: Modulation

Question 4. Separation of a data signal from a carrier wave is called_____________
Answer: Demodulation

Question 5. The refractive indices of the layers of the ionosphere are _____________
Answer: Less

Community System Assertion Types

Direction: These questions have statement 1 and statement 2. Of the four choices given below, choose the one that best describes die two statements.

  1. Statement 1 is true, statement 2 is true; statement 2 is a correct explanation for statement 1.
  2. Statement 1 is true, and statement 2 is true; statement 2 is not a correct explanation for statement 1.
  3. Statement 1 is true, statement 2 is false.
  4. Statement 1 is false, statement 2 is true,

Question 1.

Statement 1: Sky wave suffers total internal reflection in the ionosphere, whereas space wave directly penetrates the ionosphere and advance toward higher altitudes.

Statement 2: The frequency of sky waves is less than that of space waves. As frequency decreases, the refractive indices of the layers of the ionosphere also go down below the refractive index of air.

Answer: 1. Statement 1 is true, statement 2 is true; statement 2 is a correct explanation for statement 1.

Question 2.

Statement 1: If a carrier wave is amplitude modulated, the amplitude of its wave changes very slowly.

Statement 2: Generally, the amplitude of the wave of a data signal is much less than that of a carrier wave.

Answer: 2. Statement 1 is true, statement 2 is true; statement 2 is not a correct explanation for statement 1.

Question 3.

Statement 1: Space wave is rarely employed in long-distance communication.

Statement 2: A space wave propagates in a straight line. Due to the curvature of the earth, it can not advance to a long distance.

Answer: 4. Statement 1 is false, statement 2 is true,

Question 4.

Statement 1: For distant communication of audio signal, a microphone, and loudspeaker are used at the transmitting end and receiving end respectively.

Statement 2: In the communication of an audio signal, the sound wave is to be converted to a similar electromagnetic wave.

Answer: 1. Statement 1 is true, statement 2 is true; statement 2 is a correct explanation for statement 1

Question 5.

Statement 1: If a carrier wave is frequency-modulated for transmission, the rate of dissipated energy remains unchanged at the transmitting antenna.

Statement 2: If a carrier wave is frequency modulated, there is no change in its amplitude only its frequency undergoes a slow, periodic change.

Answer: 1. Statement 1 is true, statement 2 is true; statement 2 is a correct explanation for statement 1

Communication System Match The Columns

Question 1. Frequencies of a data signal lie between ω and ω’ (where (ω’ > ω) and the amplitude of the corresponding data signal is, v0 The data signal is superimposed on a carrier wave.

Communication System Signal Is Superimposed On Carrier Wave

Answer: 1-D, 2-C, 3- A, 4 – B

Question 2. For the communication system, the two columns (shown below) are to be matched

Communication System For Communication System

Answer: 1-C, 2-B, 3- A, 4 – D

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