The great advantage of the circuit shown in the figure is the fact that its frequency is controlled by a continuous voltage. This allows the circuit to be used in conjunction with digital to analog converters (DAC) by controlling the frequency either through the parallel output of a PC or by a microprocessor. The varicap can be of the double type as shown in the diagram or, failing that, two separate variacaps such as BB809 can be used, which are relatively common in our market. The coil, formed by 40 turns of wire 28 in a 1 cm diameter tube, causes the circuit to generate signals centered at approximately 3.5 MHz. The capacitors used in the circuit must be ceramic and the junction field effect transistor (JFET ) admits equivalent as the MPF102. The circuit can generate signals up to a few tens of megahertz just by changing the coil to the desired range. The width of the swept band depends basically on the characteristics of the varicap used.
