Varicap Diodes / Varactor Diodes, NXP Semiconductors Variable Capacitance Diodes, commonly known as Varicap, Varactor or Tuning diodes, are useful in many applications where a change in capacitance derived from a change in voltage is required. They are suitable for use in a wide range of application, including RF tuning, Voltage Controlled Oscillators and filters, Frequency Synthesizers and Multipliers.