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An FM broadcast station deviates \( \pm 75 \ \mathrm{kHz} \) with audio to \( 15 \ \mathrm{kHz} \), in a \( 200 \ \mathrm{kHz} \) channel. The guard band is about

  • A.50 kHz
  • B.5 kHz
  • C.20 kHz
  • D.125 kHz

Answer: C

Carson's rule gives 180 kHz, so

\[ 200 - 180 = 20 \ \mathrm{kHz} \]

leaving about 20 kHz of guard band, which accommodates the Bessel tail beyond Carson's ninety-eight per cent and any frequency error. 125 kHz would follow from comparing the channel with twice the deviation alone. 50 kHz and 5 kHz do not follow from the figures. The allocation is a good example of an engineering rule and a regulatory channel plan being designed together.

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