Example
A receiver is tuned to a Morse code signal, and the receiver's intermediate frequency (IF) is Fif = 45000 Hz. That means the dots and dashes have become pulses of a 45000 Hz signal, which is inaudible.
To make them audible, the frequency needs to be shifted into the audio range, for instance Fbaseband = 1000 Hz. To achieve that, the desired BFO frequency is Fbfo = 44000 or 46000 Hz.
When the signal at frequency Fif is multiplied by that waveform in the mixer stage of the receiver. This shifts the signal to two other frequencies: |Fif − Fbfo| and (Fif + Fbfo). The difference frequency, |Fif − Fbfo| = 1000 Hz, is also known as the beat frequency.
The other frequency, (Fif + Fbfo) = 89000 Hz, can then be removed by a lowpass filter, such as an ordinary speaker (which cannot vibrate at such a high frequency) or the human ear (which is not sensitive to frequencies over approximately 20kHz).
Fbfo = 46000 Hz also produces the desired 1000 Hz beat frequency. Using a higher or lower frequency than the IF has little consequence for Morse reception, but will invert the spectrum of received SSB transmissions, making the resultant speech unintelligible.
Read more about this topic: Beat Frequency Oscillator
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