This post shows you how to build a special circuit called a local oscillator or BFO.
This circuit is used in many electronics testers and other devices to create a specific kind of signal.
It uses a common part called a bipolar junction transistor BJT.
What is Beat Frequency Oscillator:
The beat frequency oscillator BFO finds application in receiving CW continuous wave and SSB single sideband signals.
It produces a signal that, when combined with the output of the IF amplifier in the detector forms an AM signal.
This AM signal is subsequently demodulated by the detector.
Activation of the BFO occurs when the mode switch is set to the ‘C’ CW or SSB position.
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 |
Below is a simplified circuit diagram and a brief explanation of the working of the inductance capacitance LC oscillator using a single bipolar junction transistor BJT for a beat frequency oscillator BFO.
The BJT Q1 is biased using resistor R1, ensuring proper operating conditions.
The primary winding of the 465 kHz intermediate frequency transformer L is connected to the collector of the BJT.
The secondary winding is typically used for feedback to sustain oscillations.
The configuration resembles a Hartley oscillator where the transformer and variable capacitor C1 determine the frequency of oscillation.
The inbuilt tuning capacitor of the transformer is removed, and variable capacitor C1 is used to control the tuning of the oscillator.
Adjusting C1 alters the resonant frequency.
The output signal is taken from the collector of the BJT and can be used for various applications, such as generating beat frequencies.
By adjusting variable capacitor C1 the output frequency can be set within a range of 465 kHz to 1.7 MHz.
This LC oscillator configuration provides a flexible and tunable solution for generating oscillations within a specified frequency range.
Experimentation with component values and careful tuning allows, for customization according to specific application requirements.
Formulas:
The frequency f of the LC oscillator can be determined using the formula for the resonant frequency of a hartley oscillator:
f = 1 / 2π√L * C
where,
- f: This 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 typically appears in computations involving oscillations and other periodic processes.
- L: This is the inductors measured inductance in henries H for the LC circuit, by producing a voltage inductors resist variations in current.
- C: This 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 essence, the formula says that the square root of the product of capacitance C and inductance L determines the frequency of the LC oscillator in an inverse manner.
Put more simply, an oscillator with a bigger inductance or capacitance will have a lower resonant frequency.
How to Build:
Circuit construction details mentioned below:
Transistor Biasing:
- Set up the BJT in a suitable configuration ensuring proper biasing for stable operation.
- The collector load of the transistor Q1 is connected to a tuned 465 kHz intermediate frequency transformer.
Transformer Configuration:
- Configure the intermediate frequency transformer in the traditional Hartley oscillator arrangement.
- This involves connecting the primary and secondary windings in a specific manner to achieve the desired oscillation.
Tuning Control Modification:
- Remove the inbuilt tuning capacitor of the transformer.
- Variable capacitor C1 is then utilized as the tuning control for the variable frequency oscillator.
Frequency Range Adjustment:
- The output frequency of the oscillator can be adjusted within a broad range specifically between 465 kHz and 1.7 MHz, by manipulating variable capacitor C1.
Applications:
Positioning a radio capable of detecting broadcasting range frequencies in proximity to the signal generation circuit allows for the capture of the oscillation frequency.
Adjusting the signal generator to the radios intermediate frequency produces an audible beat note.
This enables the reception of continuous wave or single sideband broadcasts with ease.
Conclusion:
By following the outlined construction steps and understanding the associated formulas, enthusiasts and engineers can create a customized inductance capacitance LC oscillator with a wide frequency range.
This oscillator functioning as a local or beat frequency oscillator, proves valuable in diverse applications such as test equipment and practical circuits.
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