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TM355 revision > B1P1 > Flashcards

B1P1 Flashcards

1
Q

What is an analogue signal

A

follows air vibrations and analogous to fluctuating air pressure

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2
Q

What is a digital signal

A

represented by different voltage levels, 1 and 0

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3
Q

What is threshold detection

A

the process of checking whether the signal is above or below some threshold

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4
Q

How is an analogue signal converted to digital

A

To convert, it is sampled first by measuring its value at regular intervals.
The values are quantised to restrict the values to a discrete set.
The next step is to encode each of the possible quantisation levels with a binary number.

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5
Q

What is a sinusoid

A

periodic signal, one that repeats are regular time intervals

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6
Q

What is a cycle

A

a section between 2 periodic points

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7
Q

What is the period

A

duration of the signal

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8
Q

What is meant by frequency

A

The number of cycles in 1 second. The unit of frequency is Hz where 1 Hz = 1 cycle per second

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9
Q

What is meant by amplitude

A

the max value of sinusoid

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10
Q

What does modulation mean

A

the message signal is converted to a suitable form for transmission

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11
Q

What is the phase

A

The point that the sinusoid has reached at a particular time

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12
Q

What are the 3 main components of fibre

A

suitable source of light, which converts electrical signal
the fibre itself
a detector, which converts light and dark back to electric signal

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13
Q

What is refractive index

A

A property of an optical medium relating the speed of light in the medium to c, the speed in a vacuum. The speed of light in the medium is given by c/n, where n is the refractive index.

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14
Q

How can light change direction in fibre

A

reflection or refraction

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15
Q

What are the properties of MM fibre

A
  • Core diameter is large
  • Light can travel in a variety of ways
  • Core diameter is commonly 50 um
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16
Q

What are the properties of SM fibre

A
  • Smaller core diameter – typically 10 um
  • Few ways wave can propagate
  • A signals travel along same path
  • Best performance over long distances
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17
Q

What is attenuation

A

when signal loses power as it travels along a transmission medium

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18
Q

What is a decibel

A

way of comparing two powers

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19
Q

Increasing the power by 3 dB…

A

doubles the power

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20
Q

Doubling of a power is a …dB increase

A

3

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21
Q

Tenfold increase of power is a …dB increase

A

10

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22
Q

What is pulse spreading

A

The smearing out of sharp transitions in a signal during the course of transmission. Pulse spreading has a number of causes, and can reduce achievable data rates.

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23
Q

How does a regenerator work

A

they counteract attenuation by restoring an optical signal to original form

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24
Q

What is a directional coupler

A

two fibres that are fused together along a short length

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25
Q

Copper wires carry multiple channels by various techniques known as…

A

multiplexing

26
Q

three transmitters send light of different colours, red green, blue along the same fibre and receiver only respond to red, green blue light respectively is known as what

A

Wavelength division multiplexing (WDM)

27
Q

What is meant by voltage

A

force that sends the current around the circuit, measured in volts

28
Q

What is current

A

measured in amperes or amps (A), the flow

29
Q

What is crosstalk

A

weak version of the signal is transferred to the other conductor pair

30
Q

What is dielectric loss

A

when small part of the energy of electromagnetic fields is wasted as heat

31
Q

What is resistance

A

caused by the fact electrons do not flow along conductors entirely freely

32
Q

Waves can be …. in various ways to produce a data carrying radio signal

A

modulated

33
Q

What is bandwidth

A

the amount of spectrum occupied by a signal, equal to the highest and lowest frequenciesw f2 –f1. The larger the bandwidth the more information the signal can convey

34
Q

What is the centre frequency

A

halfway between f1 and f2

35
Q

What is a pass band

A

the range of frequencies that the receiver responds best to

36
Q

What is meant by response

A

a measure of the relative sensitivity of the receiver to frequencies at and around the frequency it is tuned to

37
Q

What is high selectivity

A

a receiver which good at rejecting signals outside the passband

38
Q

What is the inverse square law

A

describes the reduction in power with distance from the transmitter, due to spreading.

At d = 2 ot will be ¼ of what it was a d=1 and at d=3 it will be 1/9 of what it was from d=1

A receiver that is n times as far from the transmitter will receive 1/n2 of the power

39
Q

What does isotropically mean

A

radiate equally well in all directions

40
Q

What is meant by reflection

A

light is reflected at shiny surface and travels towards and away from such as surface at equal angles

41
Q

Radio waves are … by many surfaces such as ground, building, and vehicles

A

reflected

42
Q

How can reflections cause problems

A

when the same transmission is received through 2 or more routes. This is called multipath propagation

43
Q

What is refraction

A

Radio waves can be bent by refraction

The ionosphere can bend the path of a radio wave back towards the ground

44
Q

What is absorption

A

Radio waves are absorbed as they travel through the atmosphere
Absorption is measure in decibels per meter or km

45
Q

What is meant by attenuation coefficient

A

the attenuation in decibel per km of distance travelled

46
Q

What is diffraction

A

when the spreading of bending of an electromagnetic wave when it passes through a gap or encounters a sharp corner

47
Q

What is an antenna

A

They are simply conductors that happen to be in the right shape for converting an electrical signal to a radio wave at a particular frequency

48
Q

What is a dipole

A

two conductors fed at their midpoint with an electrical signal. If it is vertical, the magnetic field acts in a horizontal direction and encircles the dipole

49
Q

What is resonance

A

a term used to describe the fact something has a preferred frequency. This creates changing currents and voltages in the antenna which increase, collapse, reverse direction as the signal goes through each cycle

50
Q

What is the near field of an antenna

A

few wavelengths from antenna, where inverse square law ceases to apply

51
Q

What is the far field of an antenna

A

after near field

52
Q

What is the beamwidth

A

the angle of a cone that contains the predominant radiation

53
Q

What is antenna gain

A

A measure that compares the performance of a directional antenna with that of a reference antenna, such as an isotropic antenna or half-wavelength dipole. It is the ratio of the power sent by the directional antenna in its preferred direction to the power sent by the reference antenna.

54
Q

What is a quarter wavelength monopole

A

half wavelength dipole, with bottom element replaced by the ground

55
Q

What is a yagi uda antenna

A

used for UHF terrestrial tv reception

56
Q

What is propagation

A

can carry radiowaves of long distances

57
Q

What is meant by a surface wave

A

follows around earths curvature, below 3 MHz

58
Q

What is meant by a sky wave

A

enable communications around the globe

59
Q

What is a multipath

A

A situation in which radio waves from a transmitter are received via two or more different paths as a result of reflection, diffraction or scattering. The multiple signals received may interfere constructively or destructively.

60
Q

What is the inverse fourth power law

A

A model relating the distance d between a transmitter and a receiver to the power received. The received power varies as 1/d4

61
Q

What are the 3 types of analogue modulation

A

AM
FM
PM

TM355 revision (11 decks)

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