Beat Frequency Oscillator (BFO)

Definition: Beat Frequency Oscillator or BFO is an oscillator that produces a sinusoidal signal of the variable audio frequency range. It provides such a frequency that can be varied over a very large range.

BFO was invented in 1901 by Reginald Fessenden. It is among the most common type of oscillator that can generate a signal of variable frequency at its output.

Block Diagram and Working of Beat Frequency Oscillator

The figure below represents the block diagram of a BFO:Here as we can see that the block diagram consists of a fixed as well as a variable frequency oscillator. Along with that it also contains 2 RF filters and a mixer that produces the combination of the two applied input frequencies. Also, an amplifier unit is present for signal amplification.

Let us now move further in order to understand the operation performed by a beat frequency oscillator.

The fixed frequency RF oscillator will generate a radio frequency signal of fixed frequency. This signal is then provided to the RF filter to have the desired range.

Also, the other oscillator generates a signal of variable frequency whose value is somewhat different as that of generated fixed frequency.

Suppose the two generated frequencies are f_x and f_y. Then these two frequencies are fed to the mixer that produces sum and difference of the two frequencies applied at its input.

The mixer present in the circuit of the BFO is adjusted in a manner that the produced difference frequency at its output will be in the audio frequency range.

Further, the received signal is provided at the input RF filter that suppresses all the other RF component except the difference component of the two frequencies.

Now the produced signal which is an audio frequency signal is fed to the AF amplifier for the amplification of the achieved signal.

Hence at the output of the amplifier, we get an amplified AF signal.

We have already discussed at the beginning of this article that the signal at the output of the BFO provides a wide variable range. Now the question arises how it can be achieved?

So, the answer to this question is that even with the slight variation in any of the two applied input frequencies, the difference output at the mixer also shows variation. This variation in the frequency of the signal can be from a few Hz to the complete audio frequency range.

Also, it is to be noted here that with the variation in the frequency, the amplitude of the signal generated remains constant.

Furthermore, the frequency produced by the two oscillators must be stable enough. As only a small change in the relative frequency will resultantly cause a large variation in difference frequency generated as output.

So, to limit the unexpected variation in the output frequency, the oscillators must possess a high degree of stability towards temperature change and variation in the supply voltage.

As we know that the BFO generates low as well as the high frequency at the output. But for this proper isolation between the two oscillators must be maintained. As the presence of any sort of coupling will generate synchronization between the two in case of small generated difference.

Resultantly, this will cause distortion in the signal achieved at the output. So, in order to minimize the chances of distortion, the voltage at the fixed oscillator must be somewhat less as compared to the voltage at the variable frequency oscillator.

The circuit of beat frequency oscillator is quite complex. Thus for simplicity purpose, nowadays, Wein bridge oscillators are generally used.

This is the key theory and working of beat frequency oscillator.