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# How to Study Communication Systems for XII-Physics?

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How to Study Communication Systems for XII-Physics?

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### How to Study Communication Systems for XII-Physics?

1. 1. 9011041155 / 9011031155 • Live Webinars (online lectures) with recordings. • Online Query Solving • Online MCQ tests with detailed solutions • Online Notes and Solved Exercises • Career Counseling 1
2. 2. 9011041155 / 9011031155 Communication System 2
3. 3. 9011041155 / 9011031155 3
4. 4. 9011041155 / 9011031155 Communication System 4
5. 5. 9011041155 / 9011031155 There are two basic modes of communication 1) point – to – point communication and broadcast. e.g. 2) Broadcast mode, i. Single analog or digital. 5
6. 6. 9011041155 / 9011031155 ii. Transmitter iii. Transducer Any device that converts one form of energy into another Electrical transducer :-a device which converts some physical variable (Pressure, displacement, temperature, force, etc.) into corresponding variations in the electrical signal at its output. 6
7. 7. 9011041155 / 9011031155 iv. Attenuation The loss of strength of a signal while propagating through a medium is known as attenuation. v. Amplification Amplification is the process of increasing the amplitude (and also strength) vi. Noise Noise is random, undesirable (unwanted) electric energy that enters the communication system vii. Receiver A receiver extracts the desired message signals from the received signals at the channel output. It 7
8. 8. 9011041155 / 9011031155 consists of a pickup antenna to pick up signal, demodulator, an amplifier and the transducer. viii. Range The maximum (largest) distance between a source and a destination upto which the signal is received with sufficient strength is termed as range. ix. Bandwidth The frequency range over which equipment operates or the portion of the spectrum occupied by the signal is referred as bandwidth. 8
9. 9. 9011041155 / 9011031155 x. Modulation The process of superimposing a low frequency signal on a high frequency wave, which acts as a carrier wave for long distance transmission is known as modulation. xi. Demodulation The process of regaining (retrieval) of information from carrier wave at the receiver is termed as demodulation. (This is the reverse process of modulation). xii. Repeater A repeater is a combination of a receiver and transmitter. Repeaters are used to extend the range of a communication. 9
10. 10. 9011041155 / 9011031155 Bandwidth of Signals Different frequency bands Extremely Low Frequency (ELF) 30 Hz to 300 Hz, Voice Frequency (VF) - 300 Hz to 3000 Hz Very Low Frequency (VLF) 3 kHz to 30 kHZ Low Frequency (LF) 30 kHz to 300 kHz Medium Frequency (MF) 300 kHz to 3000 kHz Amplitude Modulation (AM) Band High Frequency (HF) 3 MHz to 30 MHz Very High Frequency (VHF) 30 MHz to 300 MHz Ultra High Frequency (UHF) 300 MHz to 3000 MHz Super High Frequency (SHF) 3 GHz to 30 GHz 10
11. 11. 9011041155 / 9011031155 (80 to 108 MHz) Frequency Modulation (FM) Band Extra High Frequency (EHF) 30 GHz to 300 GHz Some important wireless communication frequency bands Service Frequency Comments bands Standard AM 540 – 1600 kHz broadcast FM broadcast 88 – 108 MHz Television 54 – 72 MHz VHF (Very High Frequency) 76 - 88 MHz TV 174 – 216 MHz UHF (Ultra High Frequencies) 420 – 890 MHz Cellular Mobile TV 896 – 901 MHz Mobile to base station 11
12. 12. 9011041155 / 9011031155 Radio 840 – 935 MHz Base station to mobile Satellite 5.925 – 6.425 Uplink communication GHz 3.7 – 4.2 GHz Downlink Need For Modulation base band signals. Size of the Antenna or aerial For efficient radiation and reception, the transmitting antennas (or antennae) would have lengths = 4 of frequency used. A vertical antenna of this size is impracticable and hence direct transmission of such base band signals is not practical. 12
13. 13. 9011041155 / 9011031155 3 108 15 103 20 km 4 5 km Effective Power Radiated By An Antenna According to theoretical study of radiation form a linear antenna of length „ℓ‟, the power radiated is proportional to  2 . This means, for the same antenna length, the power radiated increases with decreasing (i.e increasing frequency). Hence effective power radiated by a long wavelength base band signal would be small. For a good transmission, we need high power and hence we will use high frequency transmission. 13
14. 14. 9011041155 / 9011031155 Mixing up of signals different transmitters Sound range:- from 20 Hz to 20 kHz. So all signals from the different sources would be hopelessly and inseparably mixed up. In any city, the broadcasting stations alone would completely blanket the “air” and yet they represent a very small proportion of the total number of transmitters in use In order to separate the various signals, it is necessary to covert them all too different portions of electromagnetic spectrum. Each must be given its own frequency location. This also overcomes the difficulties of power radiation at low frequencies and reduces interference. An un-modulated carrier has constant amplitude, a constant frequency and a constant phase relationship with respect to some reference. A message consists of ever-varying quantities. Speech, for instance is made 14
15. 15. 9011041155 / 9011031155 up of rapid and unpredictable variations in amplitude (volume) and frequency (pitch). Since it is impossible to represent these two variables by a set if three constant parameters, an un-modulated carrier cannot be used to convey information, The above discussion suggests that there is a need for translating the original low frequency base band signal or information message into high frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal To achieve this, signals to be transmitted are superimposed on high frequency (small wavelength) waves called carrier waves. This process is termed as modulation, which attaches information to it. The information is then transmitted by radiating these modified carrier waves called modulated waves. The 15
16. 16. 9011041155 / 9011031155 modified carrier waves called modulated waves. The carrier wave may be continuous (sinusoidal) or in the form of pulses as shown in fig A sinusoidal carrier wave can be represented as c(t) = Ac sin (ωct + Ф) Where c (t) is the signal strength (voltage or current), Ac is the amplitude, ωc(=2πfc)is the angular frequency and Ф is the initial phase of carrier wave. 16
17. 17. 9011041155 / 9011031155 During the process of modulation, any of three parameters viz. Ac, ωc and Ф of the carrier wave can be controlled by the message or information signal. This result in three types of modulation as shown in fig. Amplitude Modulation Let c(t) = Ac sin ωct represents a carrier wave and m (t) = Am sin ωm t represents the message of the 17
18. 18. 9011041155 / 9011031155 modulating signal where ωm = 2πfm is the angular frequency of the message signal. The modulated signal cm (t) can be written as = c (t) + m (t) sin ωc t = cm (t) (Ac + Am sin ωm t) sin ωc t = cm (t) = Ac sin ωc t + uAc sin ωc t ……. (1) Where Am is the modulation index. In practice, μ Ac is kept ≤ 1 to avoid distortion. 18
19. 19. 9011041155 / 9011031155 By using sin A . sin B = 1 cos A 2 B cos A B equation (1) becomes Cm (t) A c sin c t uA c cos ( 2 uA c cos ( 2 c m) 19 t c m) t
20. 20. 9011041155 / 9011031155 Production and Detection Amplitude modulated wave Production of A.M. Wave Here the modulating signal Am sin ωm t is added to the carrier signal Ac sin ωm t to produce the signal x (t). This signal x (t) = Am sin ωt sin (t) + Ac sin ωc t is passed through a square law device which is a nonlinear device and produces an output. Y (t) = B x (t) + C x2 (t) …….. (2) 20
21. 21. 9011041155 / 9011031155 Where B and C are constants. Thus, y t B A m sin C m t B A c sin 2 A m sin2 BAm sin m m c t t sin c t m B A c sin CAm A c cos ( CAm A c cos ( c t C 2 (Am 2 ...... 3 A2 ) c m )t c c t 2 A c sin2 t 2 A m A c sin t c m) t ......... 4 1 cos2A [ sin A and sin A sin B 2 1 cos (A B) cos (A B) 2 2 21
22. 22. 9011041155 / 9011031155 In equation (4), there is a d,c term C 2 (Am 2 A2 ) and c sinusoids of frequencies ωm, 2ωm, ωc, 2ωc, (ωc – ωm) and (ωc + ωm). As shown in, this signal is passed through a band pass filter which reject d.c. and the sinusoids of frequencies ωm, 2ωm and 2ωc and retains the frequencies ωc, (ωc ∓ωm). The output of the band pass filter is same as equation and is therefore an AM wave. Detection AM wave 22
23. 23. 9011041155 / 9011031155 Common Am Applications i. AM-radio Broadcasting ii. TV picture (video) iii. Two way radio a. air-craft b. Amateur radio(SSB) c. Citizen‟s band radio d. Military 23
24. 24. 9011041155 / 9011031155 Drawback in AM 1. Low efficiency – only 20 to 30% is useful. 2. Noisy reception – AM signal is easily affected by external atmosphere and electrical disturbances. 3. Operating range is small. 4. Quality: The allowed AM bandwidth is only 10kHz and for transmission of all audio frequencies about 30 kHz bandwidth is required which affects fidelity. Due to limited bandwidth stereotype transmission is not possible. 24
25. 25. 9011041155 / 9011031155 Space Communication There are three main types of space communication. Ground wave propagation ground wave communication is used for low frequencies (500kHz to 1500kHz). This type of communication is used for medium wave radio transmission, ship communication or radio navigation. Sky wave propagation very long distance 25
26. 26. 9011041155 / 9011031155 Ne2 n' = n 1 e0m w For very low and high values of frequencies of em waves, the sky waves are either absorbed or escape from the ionosphere. Hence, following two terms are important for sky wave communication. Critical Frequency (fc) It is the maximum value of frequency of the radio waves, reflected back to the Earth from the ionosphere, when directed normally to the ionosphere. It is approximately given as, fC = g Nmax where Nmax is the maximum electron density 26 of the ionosphere.
27. 27. 9011041155 / 9011031155 Depending on the properties of ionosphere, the critical frequency changes from 5MHz to 10MHz. Space wave propagation The electromagnetic waves which travel transmitting receiving directly from antenna antenna to without being influenced by the Earth are called space waves. In this propagation, the em waves move in Earth‟s troposphere, within about 15km over the surface of the Earth. Hence, they are also known as tropospheric waves. Their frequency range is in between 30MHz to 300MHz. These waves travel in straight line. Hence, the transmitting and the receiving antenna must be in 27
28. 28. 9011041155 / 9011031155 the line of sight. But, due to the curvature of the Earth, these waves cannot be received beyond horizon. Hence, the effective reception or the range of these waves is up to line of sight only. Hence, the communication is also called line of sight communication. The figure shows curved surface of the Earth R1 B R2. At B there is a transmitting antenna of height h (BA). R is the radius of the Earth, i.e. distance of R1, B and R2 from centre of the Earth O. C is the midpoint of the line joining R1 and R2. Hence, R1 and R2 are at distance d from C. Triangles OR1A and OCR1 are right angled triangles. 28
29. 29. 9011041155 / 9011031155 ∴ OA = 2 2 OR1 + AR1 2 2 2 But, OA = R + h and AR1 = h + d 2 2 2 ∴ (R + h) = R + h + d 2 2 2 2 2 ∴ R + 2hR + h = R + h + d ∴ 2hR = d d = 2 2 2 2hR Thus, transmitting antenna is installed at the top of the mountains to increase height h to increase the range. For a 100m high antenna, the range is approximately 35km. When these waves are reflected from ground, there is a phase reversal of 180o. If the direct waves and ground reflected waves reach the receiving antenna in anti phase, they cancel each other. 29
30. 30. 9011041155 / 9011031155 Satellite Communication The electromagnetic waves having frequencies beyond 30MHz are very useful in communication because of their higher band widths. But, these waves cannot transmitted as ground waves or space waves and also as sky waves because they escape from the ionosphere. Hence, satellites communication by these waves. 30 are used for
31. 31. 9011041155 / 9011031155 The satellite receives them, amplifies them and sends them back towards the Earth, which are received by the receiving antenna. The process of sending the signal from the Earth to the satellite is called uplinking and receiving the signals from the satellite is called downlinking. The uplink and down link frequencies have atleast 2MHz frequency difference between them to avoid confusion. The transmitting and receiving antennae are tuned to the corresponding frequencies. As the geostationary satellite doesn‟t change its position relative to the Earth, there is no Doppler‟s shift in the downlink frequencies. 31
32. 32. 9011041155 / 9011031155 Global communication. Advantages 1. long distance wide spread communication. 2. A 24 hour communication is possible in remote and hilly areas with excellent quality 3. As the band width is high a large amount of information can be send at a faster rate. 4. It is cheaper and maintenance free as compared to cable communication. 5. It can be used in G.P.S. (Global Positioning System) to decide position of any object accurately. Application of remote sensing 32
33. 33. 9011041155 / 9011031155 Remote sensing A remote sensing satellite is orbiting in polar orbit at nearly 1000km from the surface of the Earth. Uses 1. In meteorology for weather forecasting, prediction of storm, snow fall etc. 2. In collection of scientific data such as changes in Earth‟s magnetic field, gravity, ionosphere etc. 3. In geological survey of underground water, oil, radioactive substances etc. 4. In military operations such as movement of troops, deployment of tanks etc. and for spying. 5. Aerial survey of flood, storm, draught affected areas. Survey of crop yields, crop diseases. 6. For finding fishing zones in sea, to observe development of forest. 33
34. 34. 9011041155 / 9011031155 7. In oceanography to study ocean temperature of the ocean surface. • Ask Your Doubts • For inquiry and registration, call 34 currents,