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PCM and Optical fibres

PCM and Optical fibres. Background information for: PH 2.2 (f) & (g). PAM and Sampling. The continuous analogue wave has its amplitude sampled Sampling must be done at a minimum of twice the rate to avoid aliasing. (a) Voltage. (b) Time. (1) Original signal. (2) PAM Signal. Quantisation.

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PCM and Optical fibres

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  1. PCM and Optical fibres Background information for: PH 2.2 (f) & (g)

  2. PAM and Sampling • The continuous analogue wave has its amplitude sampled • Sampling must be done at a minimum of twice the rate to avoid aliasing (a) Voltage (b) Time (1) Original signal (2) PAM Signal

  3. Quantisation • The amplitude (voltage) is measured and assigned to a quantum level • Each signal that falls into a level is then converted into binary code

  4. Companding • Quantised distortion will occur because each level corresponds to a range of voltages • Low-amplitude signals are more seriously affected • The quantum levels are altered by a process known as companding (compressing and expanding)

  5. TDM & FDM • Different stands of information can be sent along the same optical fibre at the same time A B A 1 B 2 1 2 • As the diagram also implies, different frequencies can be used for different signals • Other methods of multiplexing can be employed

  6. Dispersion • In a clad fibre light is dispersed • Light/signal will travel along many different paths reaching the end at different times • The signal becomes spread out • The rate of transfer is limited as the information arrives over a longer period of time Original pulses After dispersion Resultant signal

  7. Multimode fibres • To reduce multipath dispersion, multimode fibres (graded index core) were designed • With modern designs it is possible to get the rays to meet to within 1ns km-1

  8. Monomode fibres • The diameter of the fibre is only a few wavelengths thick • Light only travels parallel to the axis • Some dispersion does occur, but is very limited

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