Transcript SG Telecommunications

S.G. Telecommunications
Revision for Final Exam
What is the section about ?
•
•
•
•
•
•
•
•
Speed of Sound
Speed of Light
Wave Patterns
Frequency & Wavelength
Reflection
Diffraction
Radio Waves & Microwaves
Colour Mixing
•
•
•
•
•
Radio
Telephones
Television
Optical Fibres
Satellites
Sound and Light
• Speed of Sound is 340 metres per second
• Speed of Light is 300,000,000 m/s
• So light takes much less time to travel any
distance than sound does
• speed = distance ÷ time
Wave Terms
• ‘amplitude’ means ‘height of wave from
middle to top of crest”
• ‘wavelength’ means ‘distance from any
point on a wave to the same point on the
next wave’
• ‘frequency’ means ‘number of waves each
second’
Frequency
• To calculate frequency, use this equation
• frequency = number of waves ÷ time
• NOTE : This equation is NOT in the data
booklet
Oscilloscope Traces
A
B
Which wave is loudest ?
Which wave has the smallest amplitude ?
Which wave has the highest frequency ?
Which wave has the longest wavelength ?
C
Speed of waves
• There are two equations in the data book
which you may need
• distance = speed x time ( d = vt )
• speed = frequency x wavelength ( v = fl )
Telephone Transmitter
•
•
•
•
Microphone in mouthpiece
Turns sound energy to electrical energy
Electrical signals travel through wires
In mobile phone, electrical signals turned to
radio signals ( microwaves )
• Sent out from the aerial
• Travel through the air at speed of light
Telephone Receiver Components
•
•
•
•
•
•
Aerial - picks up many radio signals
Tuner - picks out the radio signal you want
Decoder - splits signal from the carrier wave
Power supply - supplies energy
Amplifier - adds energy to the signal
Loudspeaker- turns electrical signal into sound
TV Receiver Components
•
•
•
•
•
•
•
signal has two parts
sound signal
picture signal ( vision )
sent separately
aerial picks up both
tuner picks out both
then sound and vision
have separate components
•
•
•
•
•
decoder for each part
amplifier for each part
loudspeaker for sound
picture tube for vision
picture tube contains
electrons guns to scan the
screen
• dots on screen glow when
electrons hit them
Colour TV
• Coloured light emitted when electrons hit dots on
screen
• red, green and blue light only
• mixing red and green in equal intensities gives yellow
• mixing red and blue in equal intensities gives magenta
• mixing blue and green in equal intensities gives cyan
• mixing R, G and B in equal intensities gives white
• all switched off gives black
• mixing R, G and B in unequal intensities gives other
colours
Diffraction
• This is when waves bend round things
• Radio waves ( big l ) bend better than TV
waves and microwaves ( smaller l )
• Radio waves can follow the curve of the
earth
• TV waves and mobile phone waves only
travel ‘in line of sight’ ( straight lines )
Reflection
incident ray
normal
mirror
Always measure angles from the normal to the ray
Reflected ray is always at the same angle on the other side of
the normal
angle of incidence = angle of reflection
Satellites
•
•
•
•
•
Orbit the earth above the atmosphere
Geostationary - 24 hours above the equator
Always above same point on earth’s surface
Polar - not so high, period is smaller
Use curved reflectors to transmit and
receive signals
Curved Reflectors
This shows a
satellite receiver. It
gathers signals
over a large area
and focusses them
on to the aerial to
strengthen the
signal.
In a transmitter, the
arrows would be
reversed
Optical Fibres
•
•
•
•
Long, thin solid piece of glass or plastic
Light signals travel through it at 2 x 108 m/s
This is SLOWER than light through air
Also SLOWER than electrical signals through
metal cables
• Light undergoes Total Internal Reflection
• Many advantages : size, cost, signal capacity
• Used in Cable TV, Computer Networks
Total Internal Reflection
incident ray
incident ray
angle of
incidence
angle of
incidence
refracted ray
reflected
ray
Angle of incidence < 42 degrees, ray escapes from glass block
Angle of incidence > 42 degrees, ray reflected back into glass
Other Things to study
• Amplitude modulation and frequency
modulation
• Morse Code
• Persistence of vision ( Image retention )