Tuning circuits[edit]
Generally the use of a varicap diode in a circuit requires connecting it to a tuned circuit, usually in parallel with any existing capacitance or inductance.[2] Because a DC voltage must be applied reverse bias across the varicap to alter its capacitance, this must be blocked from entering the tuned circuit. This is accomplished by placing a DC blocking capacitor with a capacitance about 100 times greater than the maximum capacitance of the varicap diode in series with it and applying the DC from a high impedance source to the node between the varicap cathode and the blocking capacitor as shown in the upper left hand diagram, right.