Beat Frequency Oscillator(BFO) Metal Detector Design with Simulink

BFO stands for Beat Frequency Oscillator. Nowadays, machines that use BFO designs are mostly found in the lower end of the metal detector market, and are commonly sold as toys or introductory detectors for those with a budding interest in the hobby. In this blog post, it is described how a Beat Frequency Oscillator(BFO) metal detector works and how it's circuit is designed with Simulink.

A BFO design typically comprises two radio frequency (RF) oscillators that usually operate at a frequency of approximately 100 kHz. One of the oscillators contains the search coil, while the second oscillator includes a smaller coil that is concealed within the control box, and is referred to as the reference oscillator. These two oscillators are adjusted so that their frequencies are nearly identical, and their outputs are combined, or mixed, to generate sum and difference frequencies. For instance: f1 = 100.0 kHz (fixed oscillator) f2 = 100.5 kHz (search coil oscillator) The produced sum and difference frequencies are 200.5 kHz and 500 Hz, respectively.

The electronics filters out the sum of the two frequencies, leaving behind the difference signal, which conveniently falls within the audio band of 20 Hz to 20 kHz. This signal is amplified and employed to power either an internal speaker or a pair of headphones. The following shows BFO metal detector diagram drawn in Simulink.

The following picture shows the circuit diagram of Search Coil Oscillator designed with Simscape Electrical blocks in Simulink.

In radio terminology, the tone or note generated by mixing the outputs from the two oscillators, where the frequency difference is within the audible range, is referred to as a heterodyne. The frequency of the heterodyne is known as the beat frequency, thus giving rise to the term Beat Frequency Oscillator.

Bringing a metal object close to the search coil affects the coil's inductance value and slightly alters the oscillator frequency, which in turn affects the difference frequency. For instance, if a metal object causes the search oscillator frequency to change from 100.5 kHz to 100.6 kHz, the audio tone heard by the user changes from 500 Hz to 600 Hz, indicating the presence of a metal target.

BFO detectors are perfect for beginners, as they are easy to use and can be mastered quickly. They typically have only one control knob for tuning the fixed oscillator and, possibly, a second knob for adjusting the audio volume. BFO detectors are also highly sensitive across almost the entire search coil area, making them effective at pinpointing the exact location of a metal target, and they have a long battery life. Furthermore, they can discriminate between ferrous (iron-containing) and non-ferrous metals to some extent.

The BFO detector is a useful tool for coin searching while avoiding unwanted junk like nails or iron fragments. It operates by producing a high pitch for valuable, non-ferrous items and a low pitch for unwanted, ferrous items, provided that the search oscillator frequency is set higher than the reference oscillator frequency. Additionally, the BFO design is one of the easiest to implement in construction. Although it may seem like an ideal metal detector due to its ease-of-use and discriminating ability, there are downsides to its use.

One major issue with BFO detectors is their tendency to drift with small changes in component values as the temperature changes. As a result, the fixed oscillator requires frequent retuning by the user. Moreover, most BFOs operate at a relatively high frequency of 100 kHz, making them susceptible to ground capacitance. When the search coil is brought close to the ground, the difference frequency decreases. The higher the frequency of the search oscillator, the more pronounced the effect.

However, there are ways to mitigate this issue. One method is to create a Faraday shield around the search coil using a tightly bound foil screen. This adds more capacitance and slightly lowers the search frequency, but it also helps counteract the capacitance caused by the ground. To avoid acting as a short circuit turn, a small gap should be left in the Faraday shield. After winding the foil around the coil, it should be connected to the circuit ground, and the coil should be made rigid and preferably potted in epoxy compound to prevent vibration from causing the detector to go out of tune. Using a search coil with fewer turns or lowering the frequency of the search oscillator can also help reduce the effects of ground capacitance.

When using a Faraday shield with a metal detector, the detector's ability to locate metal objects is not affected even though the shield changes the search coil's inductance. However, unwanted drift caused by temperature changes can be a problem, but regulating the power supply voltage and using low temperature coefficient capacitors can help. The sensitivity and stability of the metal detector design can also be affected by the layout of the printed circuit board, which can cause coupling and instability if not designed properly.

The main challenge when designing a Beat Frequency Oscillator (BFO) is achieving stability of the two oscillators. Random frequency shifts caused by non-targets can make it difficult to keep the detector in tune.

There are two ways that a conductive target can affect the inductance of the transmit coil: the permeability effect and the eddy current effect. The permeability effect occurs with compact iron targets, while the eddy current effect is used to detect non-ferrous targets such as gold, silver, copper, and brass. However, there are some limitations to the eddy current effect that can affect discrimination capabilities.