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A receiver's intermediate filter is \( 180 \ \mathrm{kHz} \) wide at 10.7 MHz. Its fractional bandwidth is about

  • A.17 per cent
  • B.0.17 per cent
  • C.5.9 per cent
  • D.1.7 per cent

Answer: D

Dividing bandwidth by centre frequency:

\[ \frac{0.18}{10.7} = 0.017 \]

so about 1.7 per cent, which a ceramic filter achieves comfortably. The same 180 kHz at 98 MHz would be under 0.2 per cent, which is why the selectivity is provided after translation rather than at the signal frequency. That comparison is the numerical form of the superheterodyne's whole justification, and it is worth working once for each service.

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