# Physics Important Questions: Communication System

 Maharashtra State Board > Science >  Communication System > You are Here

### Very Short Answers  (1 Mark)

Q1. Video signals require higher bandwidth for transmission. Why?

• A Video signal has two components: first video and second audio. A typical TV (video) signal requires 4 MHz of bandwidth. By the time when sound is added to side band called a vestigial sideband and a little buffer space, a TV (video) signal requires 6 MHz of bandwidth.

Q2. We do not choose to transmit an audio signal by just directly converting it to an e.m. wave of the same frequency. Write two reasons for the same.

• The audio signal cannot be transmitted by just directly converting it to an e.m. wave of the same frequency due to the following reasons.
• For efficient transmission, the transmitting antennas should have length at least equal to a quarter of the wavelength of the signal to be transmitted. Thus for transmitting a low-frequency signal very long antenna is required.
• Audio frequencies are within the range of 20 Hz to 20 kHz. If transmitted directly all signals at same frequencies from different transmitters would be mixed up, creating an impossible situation to tune to any one of them. In order to separate the various signals, radio stations must broadcast at different frequencies.
• If transmitted directly the signals cannot be transmitted over long distances.
• To transmit audio signal directly large bandwidth is required.
• Note: To overcome above problems the process of modulation is used during transmission of audio signals.

Q3. Draw a plot of the variation of amplitude versus angular velocity (ω) for an amplitude modulated wave.

Where ωc = Angular speed of carrier wave

ωm = Angular speed of message wave

μ = modulation index

Ac = Amplitude of carrier wave.

Q4. What is meant by critical frequency, for sky wave propagation?

• The maximum frequency at which total internal reflection from ionosphere takes place is called critical frequency.
• Mathematically critical frequency is given by

Where fc = critical frequency and Nmax = Maximum electron density of the ionosphere

Q5. What would be the modulation index for an A.M. wave for which the maximum amplitude is ‘a’, while the minimum amplitude is ‘b’?

• The modulation index for an A.M. wave for which the maximum amplitude is ‘a’, while the minimum amplitude is ‘b’ is (a – b)/(a + b).

Q6. Why are the message signals also called baseband signals?

• Baseband refers to the original frequency range of a transmission signal before it is converted, or modulated, to a different frequency range.
• A baseband signal is a signal that can include frequencies that are very near zero, by comparison with its highest frequency. e.g. sound waveform which has a range of 20 Hz to 20 KHz is baseband signal.
• A message signal has frequencies that are very near zero, by comparison with its highest frequency. Hence message signals are also called baseband signals.

Q7. Why shortwave communication over long distance is not possible via ground waves?

• The ground wave transmission becomes weaker as the frequency of the electromagnetic wave increases hence this mode of transmission is restricted to frequencies below 1500 KHz. Shortwave communication uses frequency higher than 1500 kHz. Hence shortwave communication over long distance is not possible via ground waves

Q8. Draw a block diagram of a generalized communication system.

Q9. Explain why sky wave transmission of electromagnetic waves cannot be used for T.V. transmission.

• The sky wave propagation occurs due to the total internal reflection of the electromagnetic waves by the ionosphere.
• The electromagnets waves having frequencies less than 2 MHz are absorbed by the ionosphere whereas those having frequencies greater than 30 MHz pass through it. Hence the waves with a frequency range from 2 MHz to 30 MHz can be propagated by this method.
• The television signals have frequencies in 80-200 MHz range. Thus the TV signals are not reflected by the ionosphere due to very high frequencies and just pass through it.

Q10. A carrier wave of peak 6 V used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%?

• Given: Carrier wave peak voltage Ec = 6 V, Modulation index = μ = 75% = 0.75
• To Find: Signal wave peak voltage = Em =?

μ = (Em)/(Ec)

∴  Em = Ec x m = 6 x 0.75 = 4.5 V

Q11. State two factors by which the range of transmission of T.V. signals can be increased.

• By increasing the power of the transmitter
• Increase the height of the tower on which the transmitting antenna is mounted.
• By increasing the height of receiving antenna so that it is in the level of sight with the transmitting antenna. Matching cable and signal booster can be used.

### Short Answers  (2 Marks)

Q1. Draw a neat labelled block diagram of a detector of the amplitude modulation wave. Show the waveforms at various stages.

Q2. What are the drawbacks of amplitude modulation?

• An amplitude modulation signal is not efficient in terms of its power usage. Power wastage takes place in DSB-FC (Double Side Band – Full Carrier ) transmission.
• It is not efficient in terms of its use of bandwidth. It requires a bandwidth equal to twice that of the highest audio frequency. In amplitude modulation sidebands contain the signal. The power in sidebands is the only useful power. For 100 % modulation, the power carried by AM waves is 33.3 %. The power carried by the AM wave decreases with the decrease in the extent of modulation.
• AM detectors are sensitive to noise hence an amplitude modulation signal is prone to high levels of noise.
• Reproduction is not high fidelity. For high fidelity (stereo) transmission bandwidth should be 40000 Hz. To avoid interference the actual bandwidth used by AM transmission is 10000 Hz.

Q3. Explain the need for modulation related to the size of the antenna.

• To understand the need of modulation related to the size of the antenna, let us suppose that 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.
• For transmitting signals antenna is required whose length should be at least (λ/4), to sense the time variation of the signal.

c = υλ

∴  λ = c/υ

Length of antenna = λ/4 = c/4υ  = 3 x 108 / (4 x 20 x 103)  =  3.75 x 10m = 3.75 km

• The antenna size for considered baseband signal frequency is of impracticable length. Hence direct transmission at baseband signal frequency is not practical. If we used a frequency say 1MHz, then

Length of antenna = λ/4 = c/4υ  = 3 x 108 / (4 x 1 x 106)  =  75 m

This is a practical length of the antenna.

• Thus we can obtain transmission by use of practical antenna length using high frequency for transmission. Thus there is a need of modulation i.e. translating original low-frequency signal into high frequency before transmission.

Q4.  Explain the terms

a) Transmitter:

• A transmitter is a device which processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception.
• The important component of a transmitter in radio communication are a microphone, audio amplifiers, oscillator, modulator and antenna.

• A receiver is a device which extracts the desired message signals from the received signals at the channel output.
• The important components of a receiver in radio communication are an antenna, demodulator, amplifiers, loudspeaker.

Q5. Write a note on the bandwidth of signals.

• In a communication system, the message signal can be voice, music, picture or computer data. Each of these signals has different ranges of frequencies. 2. The type of communication system needed for a given signal depends on the band of frequencies which is considered essential for the communication process.
 Signals Frequency Range Bandwidth of signal Speech 300 Hz to 3100 Hz 2800 Hz Music 20 Hz to 20000 Hz 19980 Hz Video 1500 MHz to 1506 MHz 6 MHz Computer data 2000 MHz to 2600 MHz 600 MHz
• The important thing to be noted that irrespective of where the band is located in the frequency spectrum, it will carry the same amount of information. For example, Signal will carry the same amount of information in a frequency range from 1 MHz to 2 MHz (bandwidth 1 MHz) or the frequency range from 5 MHz to 6 MHz (Bandwidth 1 MHz)

Q6. Describe sky wave propagation in brief.

• The electromagnetic waves emitted by transmitting antenna are received after being reflected from the ionosphere are called sky waves and this type of propagation is called Skywave Propagation.

• The sky wave propagation occurs due to the total internal reflection of the electromagnetic waves by the ionosphere. The ionosphere consists of free positive and negative ions produced due to the ionization of atoms and molecules present in the atmosphere. The charged density of the ionosphere increases with height which results in the decrease in its refractive index.
• The electromagnets waves having frequencies less than 2 MHz are absorbed by the ionosphere whereas those having frequencies greater than 30 MHz pass through it. Hence the waves with a frequency range from 2 MHz to 30 MHz can be propagated by this method.
• The maximum frequency at which total internal reflection from ionosphere takes place is called critical frequency. Mathematically critical frequency is given by

Where fc = critical frequency and Nmax = Maximum electron density of the ionosphere

Q7. What do you mean by space communication?

• The electromagnetic waves emitted by transmitter antenna travel directly from the transmitting antenna to the receiving antenna are called space waves and this type of propagation is called space wave propagation. It is used for line of sight (LOS) communication and satellite communication.

• High-frequency electromagnetic waves cannot be transmitted as ground waves due to high energy losses. Moreover, these waves are absorbed by the ionosphere hence they cannot be transmitted via skywave propagation. Therefore, such high-frequency electromagnetic waves are directly transmitted throng Earth atmosphere using a transmitting antenna As these waves travel in a straight line, the receiving antenna must be in the line of sight of the transmitting antenna.
• This method is used for transmission of waves in very high frequency (VHF) band, the ultra-high-frequency band (UHF), microwaves, etc. The TV signals having frequency band 54-806 MHz can propagate neither via ground waves (due to high absorption in the atmosphere) nor via sky waves (due to non-reflection of the ionosphere). Hence TV signals can only be propagated through space wave only.

8. Explain in brief idea of the structure of the atmosphere.

• Earth is surrounded by an envelope of gases called the atmosphere. It extends to about 400 km above the surface of the earth. Its composition is not the same everywhere. The atmosphere plays an important role in the transmission of electromagnetic waves. The earth’s atmosphere is broadly divided into four different layers.

#### Troposphere:

• The layer of the atmosphere extending up to a height of 12 km from the surface of the earth is called the troposphere. This layer mostly contains water vapour which leads to the formation of clouds. The local weather changes in the earth’s atmosphere occur in this layer.
• The density of air at the surface of the earth is about 1.29 kg/m3 it decreases gradually and at the top of the troposphere, it is about 0.129 kg/m3. The temperature falls from about 15 °C to – 50 °C

#### Stratosphere:

• The region of the earth’s atmosphere lying between 12 km to 50 km is called the stratosphere. The ozone layer is part of the stratosphere extending from 15 km to 30 km; which absorbs the harmful ultraviolet radiations from the sun. Ultraviolet rays are very harmful to living cells.
• The density of air at the bottom of stratosphere 1.29 kg/m3, it decreases gradually and at the top of the stratosphere, it is about 1.29 x 10-3 kg/m3. The temperature increases from – 50 °C to 10 °C.

#### Mesosphere:

• The region of the earth’s atmosphere lying between 50 km to 80 km is called the mesosphere.
• The density of air at the bottom of mesosphere 1.29 x 10-3 kg/m3, it decreases gradually and at the top of the mesosphere, it is about 1.29 x 10-5 kg/m3. The temperature falls from 10 °C to – 90 °C.

#### Ionosphere:

• The outermost layer of the earth’s atmosphere is called the ionosphere extending from 80 km to 400 km. The Ionosphere contains charged particles and plays an important role in space communication.
• The density of air at the bottom of ionosphere 1.29 x 10-5 kg/m3, it decreases gradually and at the top of the ionosphere, it is about 1.29 x 10-10 kg/m3. The temperature rises from -90 °C to 400 °C to a height of 110 km from the surface of the earth. This region is called thermosphere.
• The ultraviolet rays and x-rays coming from sun ionize the gases in the upper layer to produce electrons and positive ions. The ionosphere is not uniform due to varying composition of the atmosphere at different heights.
 Maharashtra State Board > Science >  Communication System > You are Here