CIE A Level Physics (9702) 2019-2021

Revision Notes

20.1.1 Communication Channels

Communication Channels

  • Information can be carried along different channels of communications such as:
  • Wire pairs:
    • Wire pairs are links between information transmitters and receivers
    • They are used for low frequency, short distance communications
    • They are subject to cross-talk
    • Examples: doorbells, linking a telephone
  • Coaxial cables:
    • Coaxial cables are cables with two conducting wires in which a signal transmits through a central conductor that is surrounded by an insulator
    • There is another outer conductor which is then surrounded by a plastic cover for insulation
    • An earthed copper or aluminium braid (wire mesh) surrounds this, adding another layer of insulation to protect the signal from interference
  • Example: connecting an aerial to a television
  • Coaxial cables are much better for transmission of electrical signals than a wire pair, this is because they:
    • Have a greater bandwidth
    • Have less cross-talk
    • Radiates less electromagnetic power
  • Radio and microwave links:
    • Line of sight transmission paths to receive and transmit information at microwave and radio frequencies
  • Example: a microwave link can link a ground station to a satellite
  • Optic fibres:
    • A flexible, transparent fibre made by drawing glass or plastic
    • A signal is transmitted through total internal reflection through the core (middle) of the fibre
    • The fibre is surrounded by cladding to limit the risk of breakage
  • Examples: high speed internet, medical devices eg. endoscope

Frequencies & Wavelengths in Communication Channels

  • Wire pairs:
    • Frequencies: 10 MHz
    • Wavelengths: 30 m
  • Coaxial cables:
    • Frequencies: 2 MHz (phone), 1 GHz (TV)
    • Wavelengths: 150 m – 0.3 m
  • Radio and microwave links:
    • Frequencies: 5 GHz
    • Wavelengths: 6 cm
  • Optic fibres:
    • Frequencies: 104 GHz – 109 GHz
    • Wavelengths: 0.03 m – 3.0 × 10-7 m

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