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As NDB transmission is either N0NA1A(requires BFO to be turned on for Manual Tuning, Identification and Monitoring) or N0NA2A(BFO turned on only for Manual Tuning).

My question is what exactly is meant by Manual Tuning and how does BFO help in Manual Tuning of ADF/NDB.

Thank you

How (and when) to use the BFO button on an ADF? This post explains when to use it, But I want to understand the concept of Manual tuning and Why N0NA2A needs the BFO turned on for Manual tuning?

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  • $\begingroup$ The Beat Frequency Oscillator is a tiny little transmitter in your ADF that is tuned very close to the station that you want to receive (*). It puts a "beat", or audio note as a function of the difference between the BFO frequency and the frequency being received. A squeal if you will. If the station being received is at 380 kHz. and the BFO is tuned to 381 kHz. you will hear a 1 kHz. note, the difference between 380 and 381. $\endgroup$ – Nish May 20 '20 at 5:29
  • $\begingroup$ Even if the beacon you want to hear is modulated, there is a phenomenon called carrier enhancement that lets you hear the beat note from the BFO far before you will hear any modulation on the beacon. You can hear that squeal when the station is so far down into the noise that you can just barely detect that there is "something" there. $\endgroup$ – Nish May 20 '20 at 5:29
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Manual tuning is like the old style analogue radios with a dial or rolling switch for tuning, similar to old AM/FM radios in your car, etc. Old Automatic Direction Finder (ADF) systems still have a tuning knob that does the same thing. BFO stands for Beat Frequency Oscillator. There is a bit of electrickery going on, but basically, the ADF radio receiver processes the incoming signal and splits it in two, and just about halves one of those signal's frequencies. These two signals are then layered on top of each other so you can hear them on the headset. You then proceed to tune the radio until you minimise the pulsating sound coming from the headset. The best analogy of how this works is how a guitarist tunes by ear by comparing two strings. When they pick two strings at the same note, if they aren't in tune there is a pulsing harmonic that doesn't sound nice. When the string is tuned properly, this pulsing goes away. Same concept is happening in your ADF radio with BFO selected. So in summary, BFO help you tune to the exact NDB frequency using your own ear as a guide.

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  • $\begingroup$ Wow all these years I've wondered how BFO actually worked, having come along a bit too late to have to work a coffee grinder ADF. Is it a steady tone separate from the morse code audio? $\endgroup$ – John K May 19 '20 at 15:48
  • $\begingroup$ Hi John, So while manual tuning it gives us a Steady Tone and our aim is to keep turning the knob of the BFO until the tone is nullified. This signals that the station has been tuned. $\endgroup$ – Nish May 20 '20 at 5:29

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