English: This image depicts three methods of placing varicap diodes

in a circuit.

 The simplest method involves placing the varicap in series

with the oscillator resonator inductive element with a D.C. blocking capacitor in between, then applying the D.C. bias to the varicap via a high value resistor (47-100K) to vary its capacitance and thus the resonant frequency of the circuit.

   The second method involves using two varicaps in series, back-to-back. In this case no D.C. blocking capacitor is required, but the capacitance is halved.
   The third circuit is similar but has two seperate A.C. and D.C. "earths" coupled by a blocking capacitor.

The blocking capacitors are in the range of 10-100nF.