Voltage - Controlled Tuning using a Varactor Diode
As we explained earlier, a varactor diode which exhibits variable capacitance can be used for voltage controlled tuning.
In the circuit above, the resistance R1 is the winding resistance of the inductor. The winding resistance is in series with the potentiometer, R2.
Therefore, the winding resistance of the inductor and the potentiometer form a voltage divider that is used to determine the amount of reverse bias across the varactor diode D1 and therefore, its capacitance because an increase in reverse voltage leads to a wider depletion layer which also results to a lesser capacitance vice versa.
Thus by adjusting the setting of the potentiometer, one can vary the diode capacitance which in turn varies the resonant frequency of the LC circuit.
Resonant Frequency = 1 / 2π(LC)1/2
If the amount of varactor reverse bias is decreased the value of C of the varactor increases. The increase in C will cause the resonant frequency of the circuit to increase.
Therefore, a decrease in the reverse bias voltage causes a decrease in the resonant frequency implying that the reverse bias voltage is directly proportional to the resonant frequency and inversely proportional to the capacitance.
If the amount of varactor reverse bias is decreased the value of C of the varactor increases. The increase in C will cause the resonant frequency of the circuit to increase.
Therefore, a decrease in the reverse bias voltage causes a decrease in the resonant frequency implying that the reverse bias voltage is directly proportional to the resonant frequency and inversely proportional to the capacitance.
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