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TM355 - Communications Technology > Block 1 Part 1 Definitions > Flashcards

Block 1 Part 1 Definitions Flashcards

Channels for communication

1
Q

What is an analogue signal?

A

A signal that can take any value within a continuous range.

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2
Q

What is a digital signal?

A

A signal that can take any one of a discrete set of values.

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3
Q

What is a bipolar signal?

A

A signal that uses positive and negative voltages to represent data 1s and 0s.

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4
Q

What is threshold detection?

A

The process of checking whether the signal is above or below some threshold value, in order to decide which of its two possible values it was intended to be.

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5
Q

What is a sinusoid? (Sine wave)

A

A sinusoid, or sine wave, is a periodic signal with the general form v(t) = A sin(ωt + ϕ), where A is the amplitude, ω is the angular frequency and ϕ is the phase.

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6
Q

What is a periodic signal?

A

A signal that repeats at regular time intervals.

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7
Q

What is a cycle?

A

A section of periodic signal between two consecutive corresponding points. A periodic signal is composed of a series of repeating cycles.

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8
Q

What is period?

A

The time duration of one cycle of a periodic signal. The period T in seconds is related to the frequency f in Hz by T = 1/f.

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9
Q

What is frequency?

A

For a sinusoidal signal, the frequency is the number of cycles per unit time. The SI unit is the hertz (Hz), the number of cycles in one second.

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10
Q

What is regeneration?

A

The process of recovering the originally transmitted digital signal from a received degraded signal.

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11
Q

What is sampling?

A

Measuring an analogue signal at regular intervals in time. Sampling is part of the process of analogue-to-digital conversion.

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12
Q

What it quantisation?

A

Conversion of an analogue quantity, which could take any value within a range, to one of a set of discrete values. Quantisation is a step in analogue-to-digital conversion.

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13
Q

What is resolution in the context of analogue-to-digital conversion?

A

The number of bits in the binary output. An analogue-to-digital converter with a resolution of n bits can distinguish 2n different quantisation levels.

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14
Q

What is resolution in the context of image/video processing?

A

The number of pixels in an image or frame. Similarly, pixel resolution is the term for the number of bits (quantisation levels) used for each individual pixel - commonly expressed as bits per pixel (bpp).

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15
Q

What is amplitude?

A

The maximum value of a periodic signal. For example, the sinusoid A cos (ωt) has an amplitude of A.

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16
Q

What is a phase?

A

A description of the position of a point along a cycle of a periodic waveform. Phase is measured in units of angle, where 360° or 2π radians corresponds to a complete cycle.

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17
Q

What is a spectrum?

A

Signals or radiation arranged in order of frequency. The frequency domain.

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18
Q

What is a waveform?

A

The shape of a periodic signal in the time domain.

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19
Q

What is a harmonic?

A

One of a series of whole-number multiples of the fundamental frequency in the frequency-domain representation of a periodic waveform.

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20
Q

What is modulation?

A

The process of modifying a carrier signal in order to convey message information.

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21
Q

What is wavelength?

A

The physical length (in metres) of one cycle of an electromagnetic wave.

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22
Q

What is the core in optical fibre?

A

The innermost part of an optical fibre. It has a higher refractive index than the cladding around it.

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23
Q

What is the cladding in optical fibre?

A

A layer of an optical fibre surrounding the central core. It has a lower refractive index than the core.

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24
Q

What is total internal reflection?

A

Reflection at the interface between two optical media with different refractive indices.

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25
Q

What is multimode fibre?

A

An optical fibre with a core diameter that is large in comparison to the wavelength. It allows light to propagate in a variety of ways, unlike single-mode fibre.

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26
Q

What is single-mode fibre?

A

A type of fibre in which only one mode of propagation is possible (unlike ‘multimode fibre’), because the diameter of the core is small (e.g. 10 µm).

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27
Q

What is graded-index fibre?

A

A type of optical fibre in which the refractive index changes smoothly between the core and the cladding. Compare ‘step-index fibre’.

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28
Q

What is step-index fibre?

A

A type of optical fibre in which there is an abrupt change of refractive index between the core and the cladding. Compare ‘graded-index fibre’.

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29
Q

What is attenuation?

A

Reduction in the power of a signal, for example through absorption.

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30
Q

What is a decibel? (dB)

A

An alternative way of expressing the ratio of two powers. The decibel equivalent G of a power ratio P1/P2 is:

G = 10 log10 (P1/P2).

Because of this logarithmic relation, adding decibels is equivalent to multiplying ordinary power ratios.

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31
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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32
Q

What is multimode distortion?

A

Distortion in multimode optical fibres due to differences in the time it takes for a signal to propagate using different paths or modes. This can cause pulse spreading.

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33
Q

What is dispersion?

A

Distortion in optical fibres due to light of different wavelengths travelling at different speeds.

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34
Q

What is polarisation mode distortion?

A

Distortion in single-mode optical fibres caused by variation in the speed of light with the orientation of the light wave in the fibre.

35
Q

What is a light-emitting diode? (LED)

A

A semiconductor device that converts an electrical current to light radiation.

36
Q

What is a laser diode?

A

A semiconductor device that converts an electrical current to light. It produces a narrow beam of monochromatic infrared or visible radiation.

37
Q

What is a photodiode?

A

A semiconductor device that converts an optical signal to an electrical signal.

38
Q

What is a regenerator?

A

A device that counteracts the effects of attenuation in a digital communications link by retransmitting a corrected version of the signal it receives, with the original amplitude, shape and timing.

39
Q

What is a repeater?

A

A device that counteracts the effect of attenuation in a communications link by amplifying the signal to its original amplitude.

40
Q

What is an optical amplifier?

A

A device to increase the strength of an optical signal. It does so directly, without having to convert the light signal to an electrical signal and then back again.

41
Q

What is an erbium-doped fibre amplifier? (EDFA)

A

A type of optical amplifier in which a section of fibre is doped with a small proportion of erbium atoms and pumped by a laser.

42
Q

What is a distributed raman amplifier?

A

A type of optical amplifier in which a fibre is pumped with energy from a laser. It is called ‘distributed’ because amplification takes place along the whole length of the transmission path, not just over a short section.

43
Q

What is a semiconductor optical amplifier? (SOA)

A

A type of optical amplifier that works on the same principles as a semiconductor laser.

44
Q

What is a directional coupler?

A

An optical-fibre component used to add the signals from two fibres, or to split the signal from one fibre to two separate fibres.

45
Q

What is a combiner in optical-fibre networks?

A

A device that takes input signals from two or more optical fibres and merges them to produce a single output.

46
Q

What is a splitter in optical-fibre networks?

A

A passive device that shares an optical signal from one fibre between two or more fibres.

47
Q

What is multiplexing?

A

A technique for transmitting multiple signals along a single medium, such as a copper cable or optical fibre.

48
Q

What is wavelength division multiplexing? (WDM)

A

A technique in which one optical fibre is shared between a number of signals by using light of different wavelengths.

49
Q

What is demultiplexing?

A

The reverse operation to multiplexing; converting a multiplexed signal back to the original set of signals.

50
Q

What is a voltage?

A

The force that sends an electrical current around a circuit, measured in volts (V).

51
Q

What is a current?

A

An electric current in a cable transfers energy through the movement of electrons. Current is measured in amperes or amps (A).

52
Q

What is crosstalk?

A

Interference that occurs when a number of copper cables carrying different signals are in close proximity, so that their electric or magnetic fields can couple with each other.

53
Q

What is resistance?

A

When an electric current flows in a conductor, resistance to the flow due to collisions within the material converts some of the electrical energy to heat. Resistance is one factor contributing to attenuation in long-distance cables.

54
Q

What is dielectric loss?

A

In a copper cable, the power that is lost in the insulating material (or dielectric) that separates the conductors. Dielectric loss has the effect of attenuating the signal, particularly at high frequencies or data rates.

55
Q

What is filtering in the context of radio reception?

A

A filter allows a range of frequencies to pass through while stopping other frequencies. Similarly, an optical filter allows light of certain wavelengths (or frequencies) to pass while stopping others.

56
Q

What is bandwidth?

A

The amount of spectrum occupied by a signal; the difference between the highest and lowest frequencies.

The term is also in common usage as being loosely synonymous with the maximum data rate of a digital channel. Usually the meaning is clear from the context, but it is better to avoid this second usage if there is any chance of confusion.

57
Q

What is response (frequency response) in the context of radio reception?

A

Frequency response is a measure of the relative sensitivity of the receiver to frequencies at and around the tuned frequency.

58
Q

What is passband?

A

The range of frequencies that are passed (not blocked) by a filter; the frequency range that a radio receiver responds to most strongly.

59
Q

What is selectivity?

A

The ability of a radio receiver to reject signals outside its passband.

60
Q

What is the inverse square law?

A

A model relating the distance d between a transmitter and a receiver to the power received. The received power varies as 1/d^2.

61
Q

What is isotropic?

A

Of an antenna, one that radiates equally well in all directions.

62
Q

What is specular reflection?

A

Reflection of an electromagnetic wave at a flat surface, e.g. light in a mirror. The wave leaves at an equal angle to the one at which it entered.

63
Q

What is the ionosphere?

A

A layer of the Earth’s atmosphere containing charged particles. It can reflect, refract or absorb radio waves, depending on conditions.

64
Q

What is absorption?

A

Attenuation of electromagnetic radiation (radio waves, or light in optical fibres) resulting from its passage through a medium.

65
Q

What is an attenuation coefficient?

A

A property of a medium in the context of absorption. It expresses how much a signal is attenuated when passing one unit distance through the medium. Units are dB m^-1 or dB km^-1.

66
Q

What is diffraction?

A

Spreading or bending of an electromagnetic wave when it passes through a gap or encounters a sharp corner.

67
Q

What is a dipole?

A

A type of antenna consisting of two straight conductors arranged end to end.

68
Q

What is near field?

A

The space close to an antenna. The inverse square law breaks down in this region.

69
Q

What is far field?

A

The space starting a few wavelengths from an antenna and extending outwards. The inverse square law applies in the far field in free space.

70
Q

What is omnidirectional?

A

Of an antenna, one that radiates equally well in all directions in two dimensions, but not necessarily the third dimension. For example, a vertical dipole radiates equally well in all horizontal directions, but less well above and below the dipole.

71
Q

What is beamwidth?

A

A measure of the directional properties of an antenna. It is the angle of a cone that contains the predominant radiation, defined as radiation above a certain power level, usually 3 dB below the maximum.

72
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.

73
Q

What is a sky wave?

A

A radio propagation mechanism in which the ionosphere reflects or refracts radio waves back to Earth. This allows reception over great distances via one or more hops.

74
Q

What is a multipath reception?

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.

75
Q

What is fading?

A

Variations in received signal strength as a radio receiver moves around. Fading is additional to the gradual falling off of strength with distance described by an inverse power law.

76
Q

What is amplitude modulation? (AM)

A

A method of modulation in which a modulating signal varies the amplitude of a carrier signal. The frequency and phase of the carrier signal remain the same.

77
Q

What is a modulating signal?

A

In modulation, the signal containing the input data or message information.

78
Q

What is the envelope?

A

The shape of the modulating signal in amplitude modulation, or (approximately) the shape traced out by the peaks of the amplitude-modulated signal.

79
Q

What are sidebands?

A

The frequency components situated on each side of the carrier frequency of a modulated signal.

80
Q

What is frequency modulation? (FM)

A

A method of modulation in which a modulating signal varies the frequency of a carrier signal. The amplitude of the carrier signal remains the same.

81
Q

What is a voltage-controlled oscillator? (VCO)

A

An oscillator with a variable frequency output, where the frequency is controlled by a voltage signal. A VCO is used for FM modulation.

82
Q

What is frequency deviation?

A

The maximum deviation of the frequency of an FM-modulated signal from the carrier frequency.

83
Q

What is the modulation index?

A

In frequency modulation (FM), the modulation index b is the ratio of the frequency deviation Df and the highest frequency component in the modulating signal fm:

b = Df / fm.

TM355 - Communications Technology (2 decks)

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