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What is the Beat Frequency Oscillator?
A sine wave
Also, it’s referred to as a BFO. Essentially, it is a device that produces variable audio frequency range sinusoidal signals. It is through the combination of two varied radio-frequency electrical oscillations.
Furthermore, note that the device has been in use since its invention in 1901 by Reginald Fessenden.
Working of Beat Frequency Oscillator
The block diagram below illustrates the working principle of the device. 
Beat Frequency Oscillator Working Schematic
Note that the circuit comprises a variable-frequency oscillator and a fixed one. Also, there are 2 RF filters (RF components) and a mixer. Thus, they combine the two input frequencies applied by the fixed and variable oscillators.
In addition, there is an amplifier to facilitate signal amplification.
Working Mechanism
Radio frequencies illustration
First, the fixed frequency RF oscillator delivers a radio frequency signal. Note that this signal has a fixed frequency. Next, the signal enters the RF filter to modify the appropriate range.
Simultaneously, the variable frequency oscillator delivers a signal. Note the fixed frequency value is different from the variable one.
Now, we have two oscillator frequencies, say Fx and Fy. Next, they enter the mixer that delivers a sum and difference frequency of the oscillation frequency.
Note: The mixer will consistently deliver a different frequency in the audio frequency range.
Next, the signal travels to the input RF filter. Essentially, this filter will suppress all other RF component (s), leaving only the difference oscillator frequency.
Lastly, the AF amplifier will amplify the signal and be ready for use.
The Main Factors that Affects Beat Frequency Oscillator
A Speaker Illustrating a sound wave.
The primary factor in the operation of the device is frequency stability. Note that a slight change in the relative frequency induces a significant shift in different frequencies. Thus, reducing the possible frequency drift of the difference frequency with time is imperative.
Primarily, ensure that the individual oscillators have high inherent stability. It will be handy in withstanding the changes in temperature and the supply voltage.
Also, note that the RF oscillators must be separate from each other. Essentially, this is to caution against any form of coupling that could lead to the synchronization of the signals. It is especially common in instances where the frequency difference is minute.
Applications of Beat Frequency Oscillator
The BFO is among the most accurate devices in the detection of metals. Here’s an illustration of its use in a metal detection unit.
Metal Detector Circuit
A Guard Using a Metal Detector
The primary components include a 4093 quad Schmitt NAND IC and a search coil. Additionally, the other key parts are a switch and battery for power supplies.
The circuit creates varying signals via the resistance to rapid changes in the voltages. This reactance is important in creating a delay in the IC. The results are rapid oscillations measuring 2 MHz. A Medium Wave radio then collects the waves.
Coil Winding Specifications
The coil comprises an enameled wire modeled using the following specs:
Next, you’ll require to connect this coil to a 0V Faraday shield. Also, the insulation tape is handy here in fastening the Faraday Shield.
How to Set up the Circuit
Set the circuit as shown in the circuit diagram below.
Circuit Diagram
In addition, for the metal detector to work, it requires picking a signal on a 2MHz harmonic. When well set, it will easily detect an object such as a metal coin measuring 80 to 90 mm.
Additionally, it can easily differentiate between ferrous and non-ferrous items.
Conclusion
Above, we have covered the simple principle of a beat frequency oscillator and elucidated its primary application in the metal detector circuit. If you want to clarify the detector, reach out, and we’ll assist you immediately.
