S.G. Telecommunications

S.G. Telecommunications Revision for Final Exam

Speed of Sound Speed of Light Wave Patterns Frequency & Wavelength Reflection Diffraction Radio Waves & Microwaves Colour Mixing Radio Telephones Television Optical Fibres Satellites What is the section about ?

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 Which wave is loudest ? Which wave has the smallest amplitude ? Which wave has the highest frequency ? Which wave has the longest wavelength ?

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

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 TV Receiver Components

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 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 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 )

S.G. Telecommunications