Varicaps

Varicaps are semiconductor diodes used as capacitors. Capacitance of a p-n-junction of such diodes depends on a reverse voltage (Fig. 3.3). The connection diagram of a varicap is shown in Fig. 3.11.

Fig. 3.11. Connection diagram of a varicap

Controlled voltage in applied to a varicap through the high-ohmic resistor R to prevent shunting of varicap capacitance by a low source resistance. When the voltage V_cnt changes, the varicap reverse voltage changes, and its capacity changes accordingly.

The varicap is connected in parallel with the tuned circuit LC which is adjusted by the varicap. The stopping capacitor C_stp is connected to avoid shunting a low coil resistance around the varicap.