The Variable Frequency Oscillator (VFO) shown in the figure can generate signals in a frequency range determined by the coil and the variable. For frequencies between 1 and 5 MHz, for example, the variable may be of the common type found in midwave receivers and the coil formed of 25 + 25 turns 28 of wire on a 0.8 to 1.0 cm (2 in) ferrite rod. diameter and 12 to 15 cm in length. The circuit can generate signals up to about 30 MHz, depending only on the values of the components used in the resonant circuit. The capacitors must be ceramic and the junction field effect transistor (JFET) admits equivalents.
