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The BFO selector switch on the ADF control panel must be in the "ON" position to enable the pilot to:
  • A
    Hear the IDENT of NDBs using N0N A1A transmissions.
  • B
    Adjust the loop to the aural null position.
  • C
    Stop the loop rotation.
  • D
    Hear the IDENT of NDBs using N0N A2A transmissions.

Navigational NDBs have an unmodulated carrier wave (ICAO classification N0N).
Station identification is transmitted at roughly ten second intervals.
Older systems interrupt the carrier wave (Keying) to send an unmodulated (but also inaudible) morse code ident (classified as A1A).
User will have also erratic indications while the signal is interrupted in this type of beacon.

To make the unmodulated parts of the A1A signal audible, ADF equipment incorporates a Beat Frequency Oscillator (BF0) that produces a signal slightly removed from the received frequency that is then mixed with it.
The mixing of the two frequencies produces an audible beat frequency, tahat is the difference of the two.

Summary: selecting the BFO on makes the N0N carrier wave audible and allows the A1A type of ident to be heard.

Note: BFO is not needed for an A2A signal which is already modulated to an audible frequency.

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