Beat frequency oscillator
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A beat frequency oscillator or BFO in radio telegraphy, is a dedicated oscillator used to create an audio frequency signal from Morse code (CW) transmissions to make them audible. The signal from the BFO is then heterodyned with the intermediate frequency signal to create an audio frequency signal. A BFO may also be used to produce an intelligible signal from a single-sideband (SSB) modulated carrier by essentially reproducing the "missing" carrier. (An amplitude modulated carrier has dual complementary sidebands and thus requires twice the bandwidth and power of SSB.) SSB is widely used in amateur or "ham" radio.
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.
By varying the BFO frequency around 44000 Hz, the listener can vary the output audio frequency; this is useful to correct for small differences between the tuning of the transmitter and the receiver, particularly useful when tuning in single sideband voice. The waveform produced by the BFO beats against the IF signal in the mixer stage of the receiver. Any drift of the local oscillator or the beat-frequency oscillator will affect the pitch of the received audio, so stable oscillators are used.
For a radio signal with more bandwidth than Morse code, low-side injection preserves the relative order of the frequency components. High-side injection reverses their order, which is often desirable to counteract a previous reversal in the radio receiver.
- Paul Horowitz, Winfield Hill "The Art of Electronics 2nd Ed." Cambridge University Press 1989 ISBN 0-521-37095-7page 898