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Std 12 PHYSICS > Communication Systems > Flashcards

Communication Systems Flashcards

0
Q

Channel

A

The transmitter and receiver are connected by a physical medium called channel.

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1
Q

Communication system

A

A communication system is a device or a setup used in the transmission of information from one place to another.

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2
Q

Point-to-point mode of communication

A

Communication takes place over a link between a simple transmitter and receiver.

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3
Q

Broadcast mode of communication

A

Large number of receivers corresponding to a single transmitter

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4
Q

Signal

A

Information converted into and electric form and suitable for transmission is called a signal.

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5
Q

Analog signal

A

Analog signals are continuous variations of current or voltage.

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6
Q

Digital signal

A

Digital signals are those which can take only discrete stepwise values.

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7
Q

Transmitter

A

A transmitter converts a signal produced by a source of information into a form suitable for transmission through a channel and subsequent reception.

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8
Q

Transducer

A

Any device that converts one form of energy into another is known as a transducer.

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9
Q

Electrical transducer

A

An electrical transducer is defined as a device which converts some physical variable (pressure, displacement, temperature, force etc.) into variations in the electric signal at its output.

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10
Q

Attenuation

A

The loss of strength of a signal while propagating through a medium is known as attenuation.

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11
Q

Amplification

A

Amplification is the process of increasing the amplitude (and also the strength) of a signal by using a device called amplifier.

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12
Q

Noise

A

Noise is the random undesirable (unwanted) electrical energy that enters the communication system via the communication medium and interferes with the transmitted message.

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13
Q

Receiver

A

A receiver extracts the desired message signals from the received signals at the channel output and reconstructs a recognizable form of the original message signal for delivering it to the user of the information.

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14
Q

Range

A

The maximum distance between source and observer up to which signal is received with sufficient strength is called the range.

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15
Q

Bandwidth

A

The frequency over which and equipment operates of the portion if the spectrum occupied by the signal is called the bandwidth.

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16
Q

Modulation

A

The process of superimposing a low frequency signal on a high frequency wave which acts as a carrier for long distance transmission is know as modulation.

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17
Q

Demodulation

A

The process or regaining (retrieval) of information at the receiver is termed as demodulation.

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18
Q

Repeater

A

Repeater is a combination of receiver and a transmitter used to extend the range of communication.

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19
Q

Base band signals

A

The signals which designate (mark-out) the band of frequencies representing the original signal as directed by the source of information are termed as base band signals.

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20
Q

Unmodulated carrier

A

And unmodulated carrier has a constant amplitude, constant frequency and constant phase with respect to some reference.

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21
Q

Carrier waves

A

High frequency waves on which signals to be transmitted are superimposed are called carrier waves.

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22
Q

Three types of modulation

A
  1. Amplitude Modulation
  2. Frequency Modulation
  3. Phase Modulation
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23
Q

Amplitude modulation

A

In amplitude modulation the amplitude if the carrier wave is varied in accordance with the information (modulating) signal.

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24
Q

Frequency modulation

A

In frequency modulation, the frequency of the carrier wave is varied according to the information signal.

25
Q

Phase modulation

A

In phase modulation, phase shift of carrier wave is varied (changed) according to the information signal.

26
Q

Types of angle modulation

A

PM plus FM together are referred to as types of angle modulation.

27
Q

Detection

A

Detection is the process of recovering the modulating signal from the modulating carrier wave.

28
Q

Space communication

A

The communication that takes place through the space surrounding the earth is called space communication.

29
Q

Transmitting antenna

A

A conductor which radiates electromagnetic waves of same frequency when alternating current passes through it is called a transmitting antenna.

30
Q

Receiving antenna

A

Metallic conductor of suitable shape or size present at the receiving centre is called the receiving antenna.

31
Q

Atmosphere

A

The gaseous envelope surrounding the Earth is called the atmosphere.

32
Q

Ground wave propagation

A

When the radiowaves from the transmitting antenna propagate along the surface of the earth to the receiving antenna, the wave propagation is called ground wave propagation of surface wave propagation.

33
Q

Ground waves

A

Ground waves are the radio waves that propagate along the surface of the earth,

34
Q

Space wave propagation

A

When radiowaves from the transmitting antenna propagate around the space surrounding the Earth to reach the receiving antenna, either directly or after reflection on the ground or in the troposphere, the wave propagation is called space wave propagation or tropospheric wave propagation or line if sight propagation.

35
Q

Space waves

A

The radiowaves that travel directly from the transmitting antenna to the receiving antenna are called space waves.

36
Q

Sky wave propagation

A

When the radiowaves from the transmitting antenna propagate through the sky; to reach the receiving antenna after reflection in the ionosphere, the wave propagation is called sky wave propagation or ionospheric wave propagation.

37
Q

Angle of radiation

A

In sky wave propagation, the maximum angle made by transmitted radiowaves with the horizontal above which the radiowaves are no longer reflected by the ionosphere is called the angelic radiation.

38
Q

Critical frequency

A

The maximum frequency of transmitted radiowaves above which the radiowaves are no longer reflected back by ionosphere is called critical frequency.

39
Q

Sky waves

A

The radiowaves having frequency between 3MHz to 30MHz are reflected back from the ionosphere and are called sky waves.

40
Q

Fading

A

The disappearance of the signal for the short time due to variations in the height and density of the ionosphere is called fading.

41
Q

Name some commonly used transmission media

A

Free space and optical fibre

42
Q

What is the most commonly used wire medium?q

A

Co-axial cable

43
Q

Need for modulation

A
  1. Size of antenna or aerial
    Antenna would have to have lengths comparable to (lambda)/4 of frequency used. Vertical antenna of this height is impractical and so, direct transmission do such base a
    Band signals is impractical.
  2. Effective power radiated by antenna
    For a linear antenna of length l, the power radiated is (l/(lambda))^2. So, for the same antenna, power radiated increases with decreasing wavelength (or increasing frequency). Hence, effective power radiated by a long wavelength base band signal would be too small. So, we use high frequency signal for high power for good transmission.
44
Q

Common AM applications

A

AM radio broadcasting
TV picture (video)
Two-way radio : aircraft, amateur radio (SSB), Citizen’s band radio, military
Digital data transmission
Computer modems (used in combination with phase modulation)
NIST time signals

45
Q

Drawbacks in AM

A

Low efficiency - only 20-30% useful
Noisy reception- easily affected by external atmosphere and electrical disturbances
Operating range is small
Quality- 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 stereo sound transmission is not possible.

46
Q

What waves can be propagated through the troposphere?

A

Electromagnetic waves of FM transmitter and TV transmitter can be propagated through this layer.

47
Q

Which layer of the atmosphere receives maximum energy from the sun?

A

Ionosphere

48
Q

Why Is the ionosphere called so?

A

High energy from sun, low pressure.
Gas molecules are ionized.
Large concentration of electrons and positively charged ions.
Hence ionosphere.

49
Q

What is the thermosphere?

A

The temperature of the layer between 80-110 km in the ionosphere has temperature varying from 180 K to 700 K. This high temperature region is called thermosphere.

50
Q

What is the E-layer of the ionosphere?

A

The E- layer of the ionosphere, also known as the Kennelly Heavyside layer is a layer of high electron density beyond the thermosphere. At the end of this layer, electron density suddenly falls to zero.

51
Q

What is the F-layer of the ionosphere?

A

At a height of 250 km, there is another layer of high electron concentration known as Appleton layer or F-layer.

52
Q

What are the three main layers of the ionosphere?

A

Thermosphere
Kennelly Heavyside (E-layer)
Appleton (F-layer)

53
Q

What are the layers of the Earth’s atmosphere?

A

Troposphere
Stratosphere (ozone layer)
Mesosphere
Ionosphere (thermosphere, Kennelly Heavyside layer, Appleton layer)

54
Q

Three components of space waves

A

Direct wave
Ground reflected wave
Tropospheric wave

55
Q

Direct wave

A

Radio waves reach antenna directly (from transmitter to receiver).

56
Q

Ground reflected waves

A

Radiowaves reach the receiving antenna after reflection from the ground.

57
Q

Tropospheric wave

A

Radiowaves reach the antenna after reflection from troposphere.

58
Q

Range of line of sight propagation

A

d= (2Rh)^(1/2)
R- radius of earth
h- height of antenna

59
Q

Why is sky wave propagation least reliable?

A

Due to diurnal and seasonal changes, the variation of thickness of E or F layer and concentration of charge changes in each layer. So, sky wave propagation is least reliable.

60
Q

Critical frequency formula

A

Fc= g* ((Nmax)^(1/2))

Nmax- maximum electron density of ionosphere

Std 12 PHYSICS (12 decks)

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