Home - Blog

Beat Frequency Oscillator- Principle and Applications

An oscillator capable of producing a variable output frequency signal is essential in electronics. The beat frequency oscillator is among the best suited for such a purpose. 

We’re going to elucidate on its working frequency, applications, and other vital insights. Take a look.

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 the circuit comprises a variable frequency oscillator and a fixed one. Also, there are 2 RF filters (RF components) and a mixer. Thus, their role is to 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

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 for modification to 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. 

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 it’s essential that the RF oscillators are separate from each other. Essentially, this is to caution against any form of coupling that could lead to the synchronization of the signals. Note, it is especially common in instances where the difference in frequencies 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

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 of 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. 


Above, we have covered the simple principle of a beat frequency oscillator. Further, we have elucidated its primary application in the metal detector circuit. If you’d want to clarify the detector, reach out, and we’ll assist you immediately. 

Avatar photo
Emma Lu
Our professional engineering support saves our customers a lot of trouble and loss. >>>>>> After you place the order, our engineer will conduct technical reviews to make sure the parts can be mounted well/correctly on the boards. We will check if the component packages match well with the Gerber footprints, if the part numbers you provided match well with the descriptions, and if the polarity is clearly marked. >>>>> When your design is ready, please send your Gerber and BOM so we can quote and start!