Building a special circuit known as a local oscillator or BFO is shown in this article.
This kind of circuit is used in many electronics testers and other devices to generate a signal.
It makes use of a common circuit component called a bipolar junction transistor (BJT).
What is Beat Frequency Oscillator:
Applications for the beat frequency oscillator (BFO) include receiving SSB single sideband transmissions and CW continuous wave communications.
It generates a signal that generates an AM signal when it connects with the detectors IF amplifier output.
After that the detector removes this AM signal.
When the mode switch is in the ‘C’ CW or SSB position the BFO is activated.
Circuit Working:
Parts List:
| Category | Description | Quantity |
|---|---|---|
| Resistor | 330k 1/4 W CFR | 1 |
| Capacitors | Ceramic 68pF | 1 |
| Trimmer 500pF | 1 | |
| Semiconductor | Transistor 2N3904 | 1 |
| Transformer 465 kHz IF | 1 |
The inductance capacitance LC oscillator uses a single bipolar junction transistor (BJT) for a beat frequency oscillator (BFO) as shown in the simple circuit diagram and explanation below.
Resistor R1 is used to bias the BJT Q1 ensuring proper working conditions.
The collector of the BJT is connected to the primary winding of the 465 kHz intermediate frequency transformer L.
In order to maintain oscillations the secondary winding is normally used for feedback.
The transformer and variable capacitor C1 control the oscillation frequency in a setup that is similar to a Hartley oscillator.
To adjust the oscillators tuning the transformers built-in tuning capacitor is taken out and replaced with variable capacitor C1.
The resonant frequency changes as C1 is changed.
The BJTs collector provides the output signal which has a number of uses including creating beat frequencies.
The output frequency is set between 465 kHz and 1.7 MHz by changing variable capacitor C1.
A versatile and adjustable way to produce oscillations within a certain frequency range is provided by this LC oscillator setup.
Modification is based on ones application needs is made possible by carefully altering and component value testing.
Formulas:
The frequency f of the LC oscillator is calculated by using the formula for the resonant frequency of a hartley oscillator:
f = 1 / 2π√L * C
where,
- f is the LC oscillators frequency expressed in hertz Hz.
- 2π is the product of two times the mathematical constant pi or roughly 3.14159 it generally appears in calculations involving oscillations and other periodic processes.
- L is the inductors measured inductance in henries H for the LC circuit by producing a voltage inductors resist variations in current.
- C is the capacitance expressed in farads F of the capacitor in the LC circuit in an electrostatic field capacitors store electrical energy.
- √ The square root is represented by this symbol.
Combining everything:
In basic terms the formula says that the square root of the product of capacitance C and inductance L measures the frequency of the LC oscillator in an reverse manner.
In other words a larger capacitance or inductance will result in a lower resonant frequency for an oscillator.
How to Build:
Below are the building steps for Simple Beat Frequency Oscillator (BFO) Circuit:
Bias in Transistors:
- Make sure the BJT is properly biassed for steady operation by setting it up in the right configuration.
- A tuned 465 kHz intermediate frequency transformer is connected to the transistor Q1s collector load.
Transformer Setup:
- Set up the standard Hartley oscillator configuration with the intermediate frequency transformer.
- To achieve the required oscillation the main and secondary windings must be connected in a particular manner.
Modification of Tuning Control:
- Take off the transformers built-in tuning capacitor.
- The tuning control for the variable frequency oscillator is then applied to variable capacitor C1.
Range Adjustment for Frequencies:
- By adjusting variable capacitor C1 the oscillators output frequency gets changed across a wide range especially between 465 kHz and 1.7 MHz.
Uses:
The oscillation frequency is recorded by placing a radio that can pick up broadcasting range frequencies close to the signal output circuit.
An audible beat note is generated by setting the signal generator to the radios intermediate frequency.
This makes it possible to easily receive single sideband or continuous wave transmissions.
Conclusion:
Both engineers and hobbyists can build an adaptable inductance capacitance LC oscillator with a wide frequency range by following the provided construction methods and understanding the mentioned formulas
This oscillator which can operate as a local or beat frequency oscillator is useful in a variety of settings including testing equipment and actual circuits.
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