Beat Frequency Oscillator is a type of oscillator used to obtain variable frequency output in the audio frequency range. The basic working of this type of oscillator is as follows.
Figure 1 shows a block diagram of beat frequency oscillator (BFO). It uses two Radio frequency (RF) oscillators. One is fixed frequency oscillator and other is variable frequency oscillator. The output of both oscillators are fed as a input to the mixer. The mixer output is the signal with a frequency given by difference of two input frequencies. If fixed frequency oscillator generates a frequency of 90 kHz and variable frequency oscillator has a frequency range of 90 kHz to 110 kHz, the mixer output frequency varies between 0 to 20 kHz (90 – 90 Hz to 110 – 90 Hz). This output frequency range is in audio frequency. Afterwards, the mixer output is amplified by an audio-frequency amplifier.
1. It used to cover a very large frequency range in the audio frequency range with a single dial rotation.
2. It is become obsolete. A Wien bridge oscillator is used in its place.
1. Since it consists of two other oscillators, circuit becomes complicated compared to other oscillator.