Utilizing Potentiometers for LC Oscillator Adjustment
In the realm of LC oscillator, adjusting the frequency typically requires a variable capacitor. However, when dealing with frequencies below 100 kHz, finding a practical variable capacitor in the nanofarad range can be challenging. In such cases, a potentiometer often proves to be a viable alternative. Let’s explore an oscillator configuration using a 2.9 mH inductor from a low-energy lightbulb and a 2.7 nF capacitor (depicted in the upper circuit). The calculated resonant frequency for this combination stands at 56.9 kHz.
Flexible Operation at Low Supply Voltages
This circuit exhibits remarkable flexibility, functioning efficiently with a supply voltage as low as 1 V, thanks to its high Q factor. By introducing an additional 10 nF capacitor in parallel, the resonant frequency decreases to 26.2 kHz. However, this alteration reduces the Q factor, necessitating an increase in circuit gain. Consequently, a minimum supply voltage of 2 V becomes essential for stable operation.
Switching Between Frequencies: A Versatile Approach
One effective method to alternate between frequencies involves employing a switch, allowing seamless transitions between two distinct frequency settings (as illustrated in the middle circuit). However, for a more nuanced approach, the switch can be replaced with a 1 kΩ potentiometer, as demonstrated in the lower circuit. In this configuration, the oscillator’s frequency becomes adjustable through the potentiometer, mimicking the functionality of a 10 nF variable capacitor. It is essential to note that using a linear potentiometer can lead to a non-linear frequency adjustment. Therefore, opting for a logarithmic potentiometer ensures a smoother and more precise frequency tuning.
Addressing Damping Challenges and Maximizing Frequency Range
One notable challenge encountered is the high level of damping, resulting in energy loss that necessitates compensatory measures for maintaining stability, such as increasing the gain through a higher emitter current. This adjustment can be achieved either by reducing the emitter resistor or by elevating the supply voltage. Experiments have shown that achieving a frequency coverage beyond a 2:1 range is difficult. When capacitors with significantly different values are used, damping in the middle frequency range becomes excessive, leading to oscillation cessation. With the specified circuit values, the frequency can be finely tuned within the range of 34.2 kHz to 55.1 kHz.