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When do you have to select or not to select the BFO?
  • A
    When receiving a N0N A1A, BFO must be selected OFF.
  • B
    BFO is never required to be selected, it is only used for maintenance purposes.
  • C
    When receiving a N0N A2A, BFO must be selected ON.
  • D
    On most modern aircraft BFO is activated automatically.

LOs reference 062.02.02.01.16: 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 (frequency 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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