Notes on Communication

Semi-detailed notes on optical fibres, satellite transmission, taken for AQA Physics B, but may be suitable for other specs

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COMMUNICATION
Wireless Communications
Uses some waves in the electromagnetic spectrum.
Radio waves and some longer wavelength microwaves.
o Other wavelengths are absorbed by the atmosphere.
Atmospheric opacity
o Gamma rays, X rays and almost all UV rays are unable to pass through
the atmosphere. Visible and some wavelengths of infrared radiation
can pass through. As the wavelength increases, the waves are unable
to pass through until the longer wavelengths of microwaves and all
radio waves.
Carrier Waves
Sound waves do not propagate very far through the air.
If you have a sine wave and a transmitter you have a radio station.
However, the sine waves contain no information, so the wave needs to be
modulated to encode information onto it. This enables audio/visual signals to
be transmitted. This encodes information onto a carrier wave.
When received, the signal is demodulated by filtering out the carrier wave.
o Analogue system:
Input transducer (microphone) Modulator (combines audio
and radio frequencies Amplifier Transmitter
o Digital System:
Transducer Analogue to Digital Converter Modulator
Transmitter
Modulation
Amplitude modulation (AM):
o Information is added to the carrier wave, producing an amplitude
moderated wave (AM), which is then transmitted
o On arrival, the carrier wave is filtered, leaving information to be to the
amplifier and loudspeaker
Frequency modulation (FM):
o The information signal modifies the frequency of the carrier wave to
represent the amplitude and frequency of the information signal.

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Bandwidth
Carrier wave frequency is fc, and the audio information signal is fs.
To avoid interference with signals from other sources, an amount of
spectrum (2fs) must be allotted to the signal, fc.
Most information signals have a range of signals, either side of the carrier
wave.
In FM, there are many more sidebands, so a larger bandwidth is required.…read more

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TOTAL INTERNAL REFLECTION
What is total internal reflection?
When going from a low to a high density, you can have any angle of incidence. But,
when going from a high to a low density substance, after a certain angle of incidence
(the critical angle), total internal reflection occurs.
So...…read more

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Optical fibres
Light is continually totally internally reflected at different points along the glass fibre
Infrared or visible light electromagnetic waves are used. Gamma and X rays
penetrate glass and would escape. UV does not pass through glass.
Microwaves and radio waves have a wavelength that is too long.
The inner glass core has a higher refractive index than the cladding.…read more

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