Beat Frequency Oscillator (BFO)
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The beat frequency oscillator (BFO) is used to produce a variable frequency output in the audio-frequency (AF) range. BFO is used when the need comes to cover a very large frequency range with a single dial rotation as it can produce 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.
The BFO has mainly two RF oscillators. One of the oscillator gives a fixed frequency and the other one produces variable frequency. The variable frequency will be slightly different from the fixed frequency. The fixed and variable frequency outputs are fed to a heterodyne or mixer device. The sum and difference terms of frequencies f1 and f2 are obtained as the output of the mixer. 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.
It should be noted that the two RF oscillators are completely isolated from each other. If there is any sort of coupling 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 wave shape. 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.
The main factor that affect Beat frequency oscillators are spurious beat notes, called the whistles. These effects are produced by the mixer and are caused by cross-modulation in the AF amplifier between high order RF harmonics produced by the mixer. These spurious whistles are seen 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 has become obsolete now, because of its complicated circuit and Wien bridge oscillator has taken its place, which has already been described.