An LC oscillator is usually adjusted using a variable capacitor. However, for frequencies below around 100 kHz, this calls for a variable capacitor with a value in the nanofarad range, which is somewhat impractical. In many situations, however, a potentiometer can be used instead. We start by looking at an oscillator using a 2.9 mH inductor salvaged from a low-energy lightbulb and a 2.7 nF capacitor (see upper circuit). The theoretical resonant frequency of this combination is 56.9 kHz.
The circuit operates from a supply voltage as low as 1 V, as the resonant circuit has a high Q factor. If an extra 10 nF capacitor is wired in parallel the resonant frequency falls to 26.2 kHz. The Q factor is reduced and so the gain in the circuit must be increased, and a supply voltage of at least 2 V is needed.
Using a switch it is possible to select between the two frequencies (see middle circuit). And now the subtle bit: instead of the switch we use a 1 kΩ potentiometer (see lower circuit). In this form the frequency of the oscillator can be smoothly adjusted using the potentiometer, almost as if we were using a 10 nF variable capacitor. Experience shows that using a linear potentiometer gives a rather non-linear frequency adjustment, and so it is preferable to use a logarithmic potentiometer. A further problem is the high level of damping: the energy loss needs to be made up for with higher gain, and so with a higher emitter current. This can be achieved either by reducing the emitter resistor or by increasing the supply voltage. Experiments show that the maximum frequency coverage possible is around a 2:1 range. If the two capacitors are very different in value the damping in the middle of the frequency range is so great that oscillation stops. With the values shown in the circuit diagram, the frequency can be adjusted between 34.2 kHz and 55.1 kHz.
