A Beat Frequency Oscillator (BFO) is an electronic circuit used in radio communication to detect and extract a specific frequency from a received signal. The BFO circuit generates a signal at a specific frequency, known as the beat frequency, which is then mixed with the received signal to produce an audible signal. BFO circuits are commonly used in shortwave receivers, and they are also used in some types of metal detectors.
The basic design of a BFO circuit consists of an oscillator that generates a signal at a specific frequency, a mixer that combines the oscillator signal with the received signal, and a band-pass filter that filters out unwanted frequencies. The resulting signal is then amplified and sent to a speaker or headphones for listening.
There are several ways to design a BFO circuit, but one common method is to use a crystal oscillator. A crystal oscillator is a type of oscillator that uses a crystal to generate a precise frequency. The crystal is connected to an amplifier, and the resulting signal is mixed with the received signal.
Another approach is to use a variable frequency oscillator (VFO). A VFO is an oscillator that can be tuned to different frequencies. The VFO signal is mixed with the received signal, and the resulting beat frequency is filtered to produce an audible signal.
In both cases, the BFO circuit must be carefully tuned to the desired frequency. The oscillator must be stable and accurate, and the band-pass filter must be designed to pass only the desired frequencies.
One advantage of using a crystal oscillator is that it provides a stable and accurate signal, but it can be difficult to tune to the desired frequency. A VFO, on the other hand, is more flexible and can be easily tuned, but it may not be as stable as a crystal oscillator.
The following shows BFO metal detector circuit diagram. 
Search Coil Oscillator
In the circuit diagram, the search coil was made using 0.56mm enamelled wire, with 143 turns wound on a 5” former. This search coil has inductance of 5.5mH with a DC resistance of 4.4 ohms.The search coil can be made using the online air core inductor calculator. The search coil oscillator is designed with Colpitts oscillator configuration with the LC tank comprising of C1=560pF, C2=2.2nF and the search coil inductor L1=5.5mH. This gives oscillator frequency of fs=101.58 KHz. The amplifying part of the oscillator is made using the bipolar transistor BC108 which is biased using the voltage divider biasing method(see BJT transistor biasing).
Reference Oscillator
The reference oscillator is made using the inductor L2=4.7mH, C3=680pF and C4=2.2nF. This gives frequency of fr=101.86 KHz. Again the reference oscillator is designed using colpitts oscillator design and biased using the voltage divider biasing method which can designed using the BJT amplifier calculator.
Mixer
The signal from the search coil and the reference oscillator enters the mixer via coupling capacitors C5 and C6. The mixer circuit is made using the BC108 bipolar transistor biased with voltage divider bias. This mixer is configured in emitter modulation configuration wherein one signal is fed into the base and the other signal is fed into the collector.
Overall, the design of a BFO circuit depends on the specific application and the desired frequency. With careful design and tuning, a BFO circuit can be a useful tool for detecting and extracting specific frequencies from a received signal.
