B1P1 Flashcards
What is an analogue signal
follows air vibrations and analogous to fluctuating air pressure
What is a digital signal
represented by different voltage levels, 1 and 0
What is threshold detection
the process of checking whether the signal is above or below some threshold
How is an analogue signal converted to digital
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.
What is a sinusoid
periodic signal, one that repeats are regular time intervals
What is a cycle
a section between 2 periodic points
What is the period
duration of the signal
What is meant by frequency
The number of cycles in 1 second. The unit of frequency is Hz where 1 Hz = 1 cycle per second
What is meant by amplitude
the max value of sinusoid
What does modulation mean
the message signal is converted to a suitable form for transmission
What is the phase
The point that the sinusoid has reached at a particular time
What are the 3 main components of fibre
suitable source of light, which converts electrical signal
the fibre itself
a detector, which converts light and dark back to electric signal
What is refractive index
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.
How can light change direction in fibre
reflection or refraction
What are the properties of MM fibre
- Core diameter is large
- Light can travel in a variety of ways
- Core diameter is commonly 50 um
What are the properties of SM fibre
- Smaller core diameter – typically 10 um
- Few ways wave can propagate
- A signals travel along same path
- Best performance over long distances
What is attenuation
when signal loses power as it travels along a transmission medium
What is a decibel
way of comparing two powers
Increasing the power by 3 dB…
doubles the power
Doubling of a power is a …dB increase
3
Tenfold increase of power is a …dB increase
10
What is pulse spreading
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.
How does a regenerator work
they counteract attenuation by restoring an optical signal to original form
What is a directional coupler
two fibres that are fused together along a short length
Copper wires carry multiple channels by various techniques known as…
multiplexing
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)
What is meant by voltage
force that sends the current around the circuit, measured in volts
What is current
measured in amperes or amps (A), the flow
What is crosstalk
weak version of the signal is transferred to the other conductor pair
What is dielectric loss
when small part of the energy of electromagnetic fields is wasted as heat
What is resistance
caused by the fact electrons do not flow along conductors entirely freely
Waves can be …. in various ways to produce a data carrying radio signal
modulated
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
What is the centre frequency
halfway between f1 and f2
What is a pass band
the range of frequencies that the receiver responds best to
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
What is high selectivity
a receiver which good at rejecting signals outside the passband
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
What does isotropically mean
radiate equally well in all directions
What is meant by reflection
light is reflected at shiny surface and travels towards and away from such as surface at equal angles
Radio waves are … by many surfaces such as ground, building, and vehicles
reflected
How can reflections cause problems
when the same transmission is received through 2 or more routes. This is called multipath propagation
What is refraction
Radio waves can be bent by refraction
The ionosphere can bend the path of a radio wave back towards the ground
What is absorption
Radio waves are absorbed as they travel through the atmosphere
Absorption is measure in decibels per meter or km
What is meant by attenuation coefficient
the attenuation in decibel per km of distance travelled
What is diffraction
when the spreading of bending of an electromagnetic wave when it passes through a gap or encounters a sharp corner
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
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
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
What is the near field of an antenna
few wavelengths from antenna, where inverse square law ceases to apply
What is the far field of an antenna
after near field
What is the beamwidth
the angle of a cone that contains the predominant radiation
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.
What is a quarter wavelength monopole
half wavelength dipole, with bottom element replaced by the ground
What is a yagi uda antenna
used for UHF terrestrial tv reception
What is propagation
can carry radiowaves of long distances
What is meant by a surface wave
follows around earths curvature, below 3 MHz
What is meant by a sky wave
enable communications around the globe
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.
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
What are the 3 types of analogue modulation
AM
FM
PM
