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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
Copper wires carry multiple channels by various techniques known as...
multiplexing
26
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
Wavelength division multiplexing (WDM)
27
What is meant by voltage
force that sends the current around the circuit, measured in volts
28
What is current
measured in amperes or amps (A), the flow
29
What is crosstalk
weak version of the signal is transferred to the other conductor pair
30
What is dielectric loss
when small part of the energy of electromagnetic fields is wasted as heat
31
What is resistance
caused by the fact electrons do not flow along conductors entirely freely
32
Waves can be .... in various ways to produce a data carrying radio signal
modulated
33
What is bandwidth
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
What is the centre frequency
halfway between f1 and f2
35
What is a pass band
the range of frequencies that the receiver responds best to
36
What is meant by response
a measure of the relative sensitivity of the receiver to frequencies at and around the frequency it is tuned to
37
What is high selectivity
a receiver which good at rejecting signals outside the passband
38
What is the inverse square law
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
What does isotropically mean
radiate equally well in all directions
40
What is meant by reflection
light is reflected at shiny surface and travels towards and away from such as surface at equal angles
41
Radio waves are ... by many surfaces such as ground, building, and vehicles
reflected
42
How can reflections cause problems
when the same transmission is received through 2 or more routes. This is called multipath propagation
43
What is refraction
Radio waves can be bent by refraction | The ionosphere can bend the path of a radio wave back towards the ground
44
What is absorption
Radio waves are absorbed as they travel through the atmosphere Absorption is measure in decibels per meter or km
45
What is meant by attenuation coefficient
the attenuation in decibel per km of distance travelled
46
What is diffraction
when the spreading of bending of an electromagnetic wave when it passes through a gap or encounters a sharp corner
47
What is an antenna
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
What is a dipole
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
What is resonance
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
What is the near field of an antenna
few wavelengths from antenna, where inverse square law ceases to apply
51
What is the far field of an antenna
after near field
52
What is the beamwidth
the angle of a cone that contains the predominant radiation
53
What is antenna gain
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
What is a quarter wavelength monopole
half wavelength dipole, with bottom element replaced by the ground
55
What is a yagi uda antenna
used for UHF terrestrial tv reception
56
What is propagation
can carry radiowaves of long distances
57
What is meant by a surface wave
follows around earths curvature, below 3 MHz
58
What is meant by a sky wave
enable communications around the globe
59
What is a multipath
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
What is the inverse fourth power law
A model relating the distance d between a transmitter and a receiver to the power received. The received power varies as 1/d4
61
What are the 3 types of analogue modulation
AM FM PM

TM355 revision (11 decks)

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