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A wide band oscilloscope is terminated in 50 \( \Omega \) at its input rather than presenting one megohm because

  • A.a low input impedance reduces the noise picked up by the instrument
  • B.the vertical amplifier requires a low source impedance to remain stable
  • C.the input capacitance is then cancelled by the terminating resistance
  • D.the coaxial cable feeding it must be matched

Answer: D

At high frequencies an unterminated cable reflects, and the reflections appear on the screen as ringing that has nothing to do with the signal. Matching the cable in its characteristic impedance absorbs the wave, at the cost of loading the source with fifty ohms.

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