The beat frequency oscillator used to be the most common type of oscillator to give vari­able frequency output in the audio-frequency (AF) range. It gives a very large frequency range with a single dial rotation and, therefore, it is mostly used when it is desired to cover a very large frequency range with a single dial rotation.

The block diagram of a beat-frequency oscillator (BFO) is shown in the figure below.

Beat Frequency Audio Oscillator

It con­sists of two RF oscillators—one gives fixed frequency and the other one variable frequency and slightly different from that produced by the former. The outputs of both these oscil­lators are applied simultaneously to a heterodyning or mixer device. At the output of the mixer we get the sum and difference terms of frequencies f₁ and f₂. It is so arranged that the difference terms of frequencies f₁ and f₂ lies in the audio-frequency range. All the RF components, leaving only the audio-frequency difference component, are removed in the RF filter. Audio-frequency output is then amplified in the AF amplifier.

The practical value of a beat frequency oscillator arises from the fact that a small or moderate percentage variation in the frequency of one of the individual oscillators (such as can be had by the rotation of the shaft controlling a variable tuning capacitor) varies the beat or difference output continuously from a few Hz to throughout the entire audio-frequency range. At the same time, the amplitude of the difference frequency output is largely constant as frequency is varied.

Frequency stability of the individual oscillators is important, because a slight change in their relative frequency would cause a relatively large change in the difference frequency. To minimize the drift of the difference frequency with time, the individual oscillators should have high inherent stability with respect to variations in temperature and to supply volt­age variations.

The two RF oscillators must be completely isolated from each other. If coupling of any sort exists between them, they will synchronize when the difference is small. Hence, low values of difference frequencies are impossible to be obtained, and in addition cause interaction between the oscillators that result in a highly distorted waveshape. To reduce distortion in the output, one of the voltages applied to the mixer (preferably the one derived from the fixed frequency oscillator) should be considerably smaller than the voltage derived from the other oscillator, and preferably free from harmonics.

Beat frequency oscillators are usually affected with spurious beat notes, called the whistles. These effects are usually caused by cross-modulation in the AF amplifier between high order RF harmonics produced by the mixer. These spurious whistles often appear when the output frequency is high. Whistles can be eliminated by operating the mixer so as to minimize the generation of RF harmonics, and by using a filter and shielding to prevent the harmonics that are generated in a mixer from reaching the amplifier circuit.

Beat frequency audio-oscillator because of its complicated circuit has become obsolete now and Wien bridge oscillator has taken its place, which has already been described.