Chapter 3- Signalling

A summary of the 3rd chapter of the OCR Advancing physics textbook for AS level

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  • Created by: R_Hall
  • Created on: 03-12-12 20:08

Transverse Waves and Polarisation

  • A transverse wave is where the vibration is at right angles to the propagation of the wave (direction of travel)
  • Electromagnetic waves are transverse, but sound is a longitudinal wave
  • A polarised wave only oscillates in one direction
  • Electromagnetic radiation is made up to two transverse waves
  • A polarising filter only lets light through that passes in one direction. If two polarising filters are at right angles to each other, then no light will get through
  • Polarised light can only happen for transverse waves
  • Rotating a polarised filter in a beam of light shows the fraction of the light that is vibrating in each direction
  • Uses of polarised light- CD players use polarising crystals to transmit polarised light, skiers wear polarised sunglasses to block glare off the snow, communication satellites use different polarisations in the same frequency band to reduce interference between signals
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Sampling

  • Digital signals are represented by binary numbers, the values it can take depends on number of bits (1 byte signal= 256 values)
  • Analogue signals vary constantly
  • When you transmit a signal it will pick up noise from other signal or electrical disturbances. The receiver reconstructs the original signal to get an accurate representation of what was sent
  • Easier with digital signals as the number of values possible is limited
  • Digitising the signal- turning analogue to digital (wont be identical, but close)
  • Take the value of the signal at regular time intervals, and find nearest digital value
  • How well the digitised signal matches depends on the resolution (difference between possible digitised signals) and the sampling rate
  • Resolution is determined by the number of bits in the binary numbers representing the digital values
  • Noise limits the number of bits used for sampling
  • Minimum sampling rate is twice the maximum frequency
  • Sampling rate needs to be high enough to record all the high frequency. Low sampling rate can create low frequency signals- aliases
  • Minimum rate of sampling= 2 x maximum frequency of signal
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Signal Spectra and Bandwidth

  • Signals are made up of lots of different frequencies, the frequencies that make up a signal is its spectrum
  • All frequencies carry info, so if you lose a frequency, information will be lost
  • The range that a signal's spectrum covers is its bandwidth
  • At the radio station, the audio signal is converted into an electronic signal- this is mixed with the carrier wave- transmitted
  • All radio stations have their own carrier frequency,so they don't interfere. Also a gap between frequencies used
  • Size of gap determined by bandwidth, large bandwidth= large gap to stop overlapping
  • Rate of signal transmission depends on number of samples per second and number of bits per sample
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