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Simple Beat Frequency Oscillator (BFO) Circuit

Simple BFO circuit is small electronic circuit which make signal sound when mix with other signal.

This circuit help to hear weak radio signal like in old radio or metal detector.

It is easy to build and uses one transistor part called a bipolar junction transistor (BJT).

What is Beat Frequency Oscillator:

BFO are used in getting SSB single sideband)and CW continuous wave signal.

It make one signal that mixes with IF output from detector and make AM signal.

Then detector remove this AM signal.

When mode switches and connects to ‘C’ or SSB then BFO turns ON.

Circuit Working:

Parts List:

CategoryDescriptionQuantity
Resistor330k 1/4 W CFR1
CapacitorsCeramic 68pF1
Trimmer 500pF1
SemiconductorTransistor 2N39041
Transformer 465 kHz IF1

This LC oscillator uses one BJT transistor for make BFO.

This simple circuit and explanation show the following:

Resistor R1 gives right power to BJT Q1 so it works good.

BJT collector connect to main coil of 465 kHz transformer L.

Second coil give feedback to keep oscillation going.

Transformer and variable capacitor C1 set the frequency like in Hartley oscillator.

To change tuning we remove transformers built-in capacitor and use C1 instead.

When C1 value is changed frequency also changes.

BJT collector give output signal and this signal is used for beat frequency and more.

Output frequency goes from 465 kHz to 1.7 MHz by moving C1.

This setup gives easy way to make signal in that range.

We can change parts to fit what we need by testing and adjusting the values.

Formulas:

The frequency f of LC oscillator is found using this formula from Hartley oscillator:

f = 1 / 2π√(L × C)

where:

This formula say: if L or C become big then frequency f become small.

So more capacitance or more inductance = lower frequency.

Less capacitance or inductance = higher frequency.

How to Build:

Below mentioned are the building steps for Simple Beat Frequency Oscillator (BFO) Circuit:

Bias in Transistor:

Transformer Setup:

Tuning Control Change:

Frequency Range Change:

Uses:

Conclusion:

Engineers and hobby people can build this Simple Beat Frequency Oscillator (BFO) Circuit easily.

It has wide frequency range and can work as BFO or local oscillator.

This circuit is good for testing or using in actual circuit.

References:

Beat frequency oscillator

BFO Schematic Diagram and Circuit Description

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