CBSE Class 12 Physics Communication Systems Notes Set 03

10. COMMUNICATION SYSTEMS

GIST

1. COMMUNICATION

The sending and receiving of message from one place to another is called communication. Two important forms of communication systems are (i) Analog and (ii) digital.

In analog communication the signal is continuous while in digital communication the signal is discrete.

2. THREE BASIC ELEMENTS OF COMMUNICATION

(i) Transmitter (ii) Communication channel (iii) Receiver

3. MODULATION

The superposition of (audio frequency) message signal (20 Hz-20 kHz) over (high frequency) carrier wave (≈ 1MHz) is called modulation.

4. NEED FOR MODULATION:

* Size of antenna h= λ/4 so, for high frequency. Height will be large which is impossible.

* Effective power radiated by an antenna P α 2 1

 * Mixing up of signals from different transmitters.

5. TYPES OF MODULATION

There are two broad types of modulation: (i) Continuous wave modulation (ii) Pulse modulation.

1. Continuous wave modulation is of three types:

(i) Amplitude modulation (AM): In amplitude modulation, the amplitude of carrier wave varies in accordance with instantaneous voltage of information (or message) signal.

(ii) Frequency modulation (FM): In frequency modulation the frequency of carrier wave is varied in accordance with instantaneous voltage of information signal.

(iii) Phase modulation (PM): In phase modulation, the phase of carrier wave is varied in accordance with the information signal.

6. Amplitude  Modulation:

7. Space Communication:

Space Communication use free Space between transmitter and receiver Space Communication is via

1.ground wave      2.Space wave      3.  Sky  wave      

8. GROUND OR SURFACE WAVE PROPAGATION is a mode of wave propagation in which the ground has a strong influence on the propagation of signal wave from the transmitting antenna to receiving antenna .In this propagation ,the signal waves glides over the surface of earth, Ground waves are heavily absorbed by earth and not suitable for long range communication. Ground wave propagation can be sustained only at low frequencies (500 kHz-1500 kHz).

9. SKY WAVE PROPAGATION is a mode of wave propagation in which the radiowave emitted from the transmitter antenna reach the receiving antenna after reflection by ionosphere. Sky wave propagation is possible because of reflection of carrier signals from ionosphere or satellite.

10. SPACE WAVE PROPAGATION higher than 30MHz is that mode of wave propagation in which the radiowaves emitted from the transmitter antenna reach the receiving antenna through space. These radiowaves are called space waves.a

is also called line of sight communication. Space wave is suitable for UHF/VHF regions.

Band width of the signal

11. COVERING RANGE OF T.V. TRANSMITTING TOWER is d=√2Reh, where h is height of tower and Re radius ofearth. T.V. waves are frequency modulated waves. VHF T.V. waves range from 47 to 230 MHz and UHF T.V. waves have range from 470 to 960 MHz.

Maximum line of sight distance d_m =√2Rh_T + √2Rh_R.

12. MAXIMUM USABLE FREQUENCY

It is that highest frequency of radio waves which when sent at some angle towards the ionosphere, gets reflected from that and returns to the earth.

13. SATELLITE COMMUNICATION

The communication satellite is used for reflecting sky waves without any disturbance. Its height is 35800 km above earth’s surface. To cover entire globe of earth simultaneously 3-satellites are employed.

II. IMPORTANT FORMULAE

1. Marconi antenna is grounded, and its length = λ/4, where λ is wavelength of the waves transmitted. It is called quarter wave antenna.

2. Hertz antenna is not grounded, and its length = λ/2. It is called half wave antenna.

3. Side band frequencies in AM wave are υSB = υc ± υmwhere υm is frequency of modulating (audio) signal.

4. Modulation index, m_a = E_m / E_c

Modulation index, m_a= E_max – E_min / E_max + E_min

6. Coverage range (d) for a given height (h) of antenna

d = √2hR where R = radius of earth.

d = √2Rh_r + √2RhR, where hT , hR are the heights of transmitter and receiver antennas.

7. Population covered = population density x area covered.

8. Number of channels,

N = Total band width of channels / Band width per channel

III. Communication System – Block Diagrams

QUESTIONS

ELEMENTS OF COMMUNICATION SYSTEMS

1. Mention the functions of the transponder? 

Ans: A device fitted on the satellite which receives the signal and retransmits it after amplification.

2. What should be the length of dipole antenna for a carrier wave of 5 x 108 Hz? (1)

Ans: L = λ\2 = c\v x 2 = 3 x 108 / 5 x 108 x 2 = 0.3m.

3. *A device X can convert one form of energy into another. Another device Y can be regarded as a combination of a transmitter and a receiver. Name the devices X and Y. 

(a) Transducer (b) Repeater

4. Name the two basic modes of communication. Which of these modes is used for telephonic communication? 

HINT:

Please refer to the link below for CBSE Class 12 Physics Notes - Communication Systems(1)