Beat frequency oscillator
Encyclopedia
A beat frequency oscillator or BFO in radio telegraphy, is a dedicated oscillator used to create an audio frequency signal from Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...

(CW
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...

} transmissions to make them audible. The signal from the BFO is then heterodyne
Heterodyne
Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...

d with the intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...

signal to create an audio frequency signal. A BFO may also be used to produce an intelligible signal from a single-sideband
Single-sideband modulation
Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....

(SSB) modulated carrier by essentially reproducing the "missing" carrier. (An amplitude modulated
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...

carrier has dual complementary sidebands and thus requires twice the bandwidth
Bandwidth
Bandwidth is the difference between the upper and lower frequencies in a contiguous set of frequencies. It is typically measured in hertz, and may sometimes refer to passband bandwidth, sometimes to baseband bandwidth, depending on context...

and power of SSB.) SSB is widely used in amateur or "ham"
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...

## Example

A receiver is tuned to a Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...

signal, and the receiver's intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...

(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 do that, the desired frequency shift is Fbfo = 44000 Hz.

When the signal at frequency Fif is multiplied by that waveform in the mixer
Frequency mixer
In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...

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.