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A Level Physics > Telecommunication > Flashcards

Telecommunication Flashcards

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1
Q

What are the different channels through that information can be carried?

A
  1. Wire pairs - electric currents
  2. Coaxial cables
  3. Radio and microwave links
  4. Optic fibres
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2
Q

What are wire pairs?
What are they used for?
What is the disadvantage?
What are the examples?

A

They are links between information transmitters and receivers.
They are used for low frequency, short-distance communications.
They are subject to cross-talk
doorbells, linking a telephone

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3
Q

What does AM modulation mean?

A

The amplitude of the carrier wave is varied in synchrony with the displacement of the information signal.

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4
Q

What is the bandwidth?

A

2 x fs

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5
Q

What are sidebands?

A

a sideband is a band of frequencies higher than or lower than the carrier frequency, resulting from the modulation process.
The sidebands carry the information transmitted by the radio signal.

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6
Q

What does AM Bandwidth mean?

A

It is the range of frequencies occupied by the amplitude modulated waveform.

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7
Q

Why is a transmitted wave modulated?

A
  1. Less interference
  2. Shorter aerial - carrier frequency is high
  3. Increased range - less attenuation
  4. Less distortion
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8
Q

What is FM modulation?

A

The frequency of the carrier wave is made to vary in synchrony with the displacement of the information signal.

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9
Q

Describe the transmission of AM signals.

A

AM signals are reflected back to the earth from the ionosphere layer.
They have a large range of coverage and one transmitter can cover a wide area.

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10
Q

Describe the transmission of FM Signals.

A

At a higher frequency, FM modulated signals pass through the ionosphere and do not get reflected. Hence they do not have as much coverage (around 30 km) and line of sight So to cover a larger area, many transmitters are required.

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11
Q

What is the Bandwidth and broadcast frequency of AM signals?

Given this, what is a suitable use?

A

bandwidth -> 9 kHz
Highest broadcast frequency -> 4.5 kHz
Best use: Speech not music

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12
Q

What is the bandwidth and broadcast frequency of FM signals?
Given this, what is their suitable use?

A

Bandwidth -> 200 kHz
Highest broadcasting frequency -> 15 kHz (about the human hearing range)
Best use: Music

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13
Q

What are the Pros and Cons of an AM transmission?

A

Pro:
- Smaller bandwidth - can accommodate more stations
- Cheaper - simpler circuitry
Cons:
- Noise
- High power transmission - changing amplitude

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14
Q

What are the Pros and Cons of an FM transmission?

A 
Pro: 
- Higher quality and less noise 
- Lower power transmission 
Cons:
- larger bandwidth - fewer stations 
- Expensiver - complex transmitters
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15
Q

What is a skywave?

A

They travel in the atmosphere in straight lines, reflecting back and forth between the ionosphere and the earth’s surface and hence goes a long distance

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16
Q

What are the disadvantages of the skywaves?

A
  1. Unreliable due to varying thickness, density and height of the ionosphere
  2. Narrow bandwidth - less information
  3. Coverage is limited and poor in hilly areas.
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17
Q

What are ground waves?

A

They travel close to the earth surface and diffract easily due to the longer wavelength.

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18
Q

What is a space wave?

A

It passes through the ionosphere

line of sight

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19
Q

What are the frequency ranges of:

i. space wave
ii. Skywaves
iii. ground waves

A

i f > 30 MHz
ii 3 < f > 30 MHz
iii f < 3 MHz

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20
Q

What is an analogue signal?

A

It is an information signal that has the same variations with time as the information itself.

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21
Q

What is a digital signal?

A

It is a series of highs and lows with no values between the highs and lows
Discrete amplitudes

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22
Q

What is noise?

A

It is the unwanted random signals that are superposed on the transmitted signals

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23
Q

What is attenuation?

A

It is the loss in signal power/intensity as it travels through the signal transmission line

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24
Q

What are the cons of an analogue wave?

A

When transmitted over a long distance, it will pick up noise and signal strength is attenuated and will need to be amplified at regular intervals

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25
Q

Why are digital signals better for transmission than analogue signals?

A
  1. Signals can be regenerated to return the wave t its original form.
  2. Signal will not be degraded
  3. More data can be added to the signal
  4. Encryption
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26
Q

What is meant by the regeneration of the signal?

A

The noise/distortion is removed from the information signal.
The original signal is reproduced/ restored.

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27
Q

Explain why an analogue signal cannot be regenerated.

A

Noise is superimposed on the digital signal’s displacement and cannot be distinguished between the noise and the signal, because the analogue signal is continuous.
An hence, the noise is amplified together with the signal.

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28
Q

What are the stages of the conversion of Analog to Digital?

A
  1. Sampling for Quantization.

2. Encoding: converting it to binary number

29
Q

Describe the process of Analog-to-digital conversion.

A

The analogue voltage is sampled at regular intervals of time, at what is known as sampling frequency or sampling rate, The value of the sampling voltage is measured at each sampling time and it is converted into a digital (binary) number that represents the voltage value.

30
Q

What does the parallel-to-serial converter do?

A

It takes the simultaneous bits of the binary number and transmits them one after the other down a single transmission line.

31
Q

What are the advantages of transmitting information in digital rather an in analogue form?

A
  1. The signal can be regenerated to remove the noise since digital is a series of discrete numbers
  2. Signal can be encrypted for security
  3. Multiplexing - data can be sent simultaneously
  4. Digital signals are cheaper and more reliable -op amp.
32
Q

How can the reproduction of the input signal be improved?

A
  1. Take more frequent samples so that the width of the steps is reduced.
  2. Increase the number of bits in the digital number to reduce the step height.
33
Q

What is the advantage of recording the input data with more bits?

A

smaller step height means that smaller changes in the signal can be represented/ reproduced
The input signal can be more accurately reproduced.

34
Q

What is the benefit of using a long word length rather than a shorter word length? [1]

A

Smaller changes in the amplitude can be detected and recorded

35
Q

What is the bit rate?

A

Bit rate = sample rate x bit depth

36
Q

The structure and uses of a wire pair

A
  • Potential difference between 2 wires is the signal
  • Wire acts as an aerial
  • Modern communication: Short-distance communication at low frequencies
  • Examples: Microphones & Doorbells
37
Q

Advantages of wire pair

A
  1. Suitable for transmission of speech
  2. media transmitted can be both analogue and digital
  3. Least expensive/ costly to replace
  4. A damaged portion does not shut down the entire network.
38
Q

Disadvantages of wire pair

A
  1. Crosstalk will occur )setting up emf adjacent wire)
  2. System is less secure
  3. Signal attenuation is large
  4. Limited bandwidth
  5. Thin - easy to break
  6. Range: 5 km before regeneration
39
Q

What are the parts of a Coaxial cable?

A
  1. Plastic coating
  2. Thin wire braid
  3. Inner metal conductor
  4. Insulator
40
Q

What is the purpose of the coaxial cable?

A
  1. Earthed and acts as a return for the signal
  2. Shield the conductor from interference/cross talk
  3. To connect between aerial and TV
41
Q

Ad of Coaxial cable

A
  1. Less crosstalk - less distortion and attenuation due to the copper braiding
  2. Better security, not easily tapped
  3. Can span longer distance at higher data per second
  4. Can transmit both digital and Analog signals
  5. immune to em interference
  6. Radiate less em power
  7. Greater bandwidth
  8. Lower noise levels
42
Q

The disadvantages of Coaxial cable

A
  1. More expensive compared to wire pair
  2. Thick cable
  3. Range: 10 km before regeneration
43
Q

Structure of optical fibres

A
  1. Glass core
  2. Glass Cladding
  3. Primary coating
  4. Outer Jacket
44
Q

advantages of optical fibres/ underseas cables

A
  1. Can send a large amount of data by a large distance
  2. Range is 80 km before regeneration
  3. Digital signals only as the unwanted signal can be removed.
  4. Not affected by power surges, electromagnetic interference, corrosion
  5. LIghter that copper
  6. Difficult to tap and higher security, no crosstalk.
45
Q

Disadvantages of optical fibres/ undersea cables

A
  • Cost of incorporating equipment for regeneration (amplification)
  • Optical transmission is one direction, two fibre cables required for two way communication.
46
Q

How do radio wave and microwave links work?

A

Radiofrequency is radiated by an antenna and produced by alternating current fed to the antenna
Microwave frequency - point to point/ line of sight transmission.

47
Q

Advantages of radio wave links?

A
  1. Choices of aerial determines whether the radio wave is emitted in all directions (radio broadcast) or in one direction (point to point communication - line of sight)
  2. Aerials with dish allow the stronger wave to be transmitted in a parallel beam.
  3. Security achieved by data encryption - parallel beams are narrow
48
Q

Disadvantages of radio wave links?

A

Radiowaves attenuates depending on the frequency

49
Q

Problem with long-distance communication using radio waves.

A
  1. Unreliable
    - affected by reflections from layers of ions in the upper atmospheres
    - affected by poor reception in hilly areas
  2. Wavebands are already crowded
  3. Bandwidths available are narrow and unable to carry large amounts of information.
50
Q

Describe how the satellite works

A
  1. Transmitter sends the wave - uplink
  2. Satellite receives the greatly attenuated signal
  3. The signal is amplified and then transmitted back down to earth.
51
Q

Why does the uplink frequency have to be different from the downlink frequency?

A

To prevent swamping between the uplink and downlink frequency

52
Q

What are the different possibilities for uplink and downlink frequencies?

A

Uplink (GHz) — Downlink (GHz)
6 4
14 11
30 20

53
Q

What are the 2 possible orbits that a satellite can have?

A
  1. geostationary

2. Polar

54
Q

Characteristics and period of a geostationary satellite.

A

Appear to remain above the same point on the earth’s surface from the viewpoint of a person on earth
24 hrs

55
Q

Characteristics and period of a polar orbit

A

pass over poles at a lower orbit

100 minutes

56
Q

Advantages of geostationary satellite.

A
  1. Aerials can be fixed in the position since the satellite does not need to be tracked
  2. Can be permanently linked to the ground station to maintain communication
  3. Removing the need for long-distance cables
57
Q

Advantages of a polar satellite.

A
  1. Greater resolution
  2. Used for spying, mapping (GPS) and remote sensing
  3. Delays in signal transmission are not noticeable
58
Q

Disadvantages of a geostationary satellite.

Solution?

A
  1. Communication with polar regions is not possible as it is not within the line of sight
  2. Height of the orbit is large causing signal attenuation and time delays - 0.24s

Solution: Use in conjunction with Fiber optics
- expensive to launch due to its high orbit & high transmission power

59
Q

Disadvantages of a polar satellite.

A
  1. Needs to be tracked by aerials in the dish
  2. Information needs to be stored in the satellite and transmitted back to the earth when the satellite is over the appropriate area - consistent coverage is not possible
60
Q

What do we need to detect a signal?

A

The signal to noise ratio/ power ratio must be large enough

61
Q

What is a decibel?

A

10 x lg(P2/P1)

62
Q

Why is log 10 used in the calculations of decibels?

A
  1. log 10 provides smaller numbers to work with

2. The gain of the amplifier in series is found by addition (of logs) instead of multiplication

63
Q

What is the signal to noise ratio?

A

At the transmitter: 10 x log(Pt/Pn)
At the reciever: 10 x log(Pr/Pn)
Add them up

64
Q

What are the typical wavelengths for:

  1. Sky waves
  2. Ground (surface) waves
  3. space waves
A
  1. 10 -> 100 m
  2. More than 100
  3. Less than 100
65
Q

What are the maximum transmission ranges for:

  1. Sky waves
  2. Ground (surface) waves
  3. space waves
A
  1. Worldwide
  2. 1000 km
  3. line of sight/ worldwide using satellites
66
Q

Explain the role of the base station and the cellular exchange when a mobile phone is switched on and before a call is made or received.

A
  1. Phone transmits a signal (to identify itself)
  2. Signal received by several base stations
  3. Transferred to cellular exchange
  4. Computer selects the base station with the strongest signal
  5. Assigns a (carrier) frequency
67
Q

State 2 changes that occur in a signal as it is transmitted along an optic fibre

A
  1. Signal becomes distorted and noisy

2. Signal loses power/ energy/ intensity / is attenuate

68
Q

What is the role of the copper braid in a coaxial cable?

A

1 - Acts as a return for a sign

2- Shields the inner core from noise/ cross talk

A Level Physics (16 decks)

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