BFO stands for Beat Frequency Oscillator. When switched on it allows the operator to identify a NDB that has no modulated ident. Most NDBs consist of a continuous carrier amplitude modulated with an ident, usually at 1020Hz (A3A).
For these BFO off would be normal, however there exist NDBs that ident by defining their ident by simple on and off keying of the carrier (A1A), for these the BFO is needed to beat with the carrier and provide an audio tone that the ear can cope with to positively indentify.
Edit for spelling and improved clarity, posting intoxicated again.
Last edited by Wodrick; 30th July 2006 at 08:51.