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The BFO selection will:
  • A
    cause the ADF to automatically select the best frequency of nearby NDBs.
  • B
    make the modulation tone audible.
  • C
    make the carrier wave audible.
  • D
    increase the transmission signal of the NDB.

LOs reference State that on modern aircraft, the BFO is activated automatically.

Beat Frequency Oscillator (BFO)

The purpose of a BFO is to allow an unmodulated transmission such as N0N/A1A to be heard. It works by adding a beat frequency to the frequency received from the NDB. These two frequencies are inputs to a heterodyne (frquency mixing unit). The mixing of the two frequencies produces an audible beat frequency, which is the difference of the two.

On older equipement, there is a separate BFO button that can be pressed in order to identify unmodulated NDB signals. In modern aircraft, the BFO circuit has no push button, as the BFO is activated automatically when required.

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