Every activity in the world is designed to work on a time reference. Timing is important for determining a relationship between events and activities in the world. A method to have a time reference in the electronic design is Clock. A clock is a signal used to provide the time reference for all the activities, events and order in which the execution has to take place. Controlled Oscillators are an important cog-in-the-wheel of the timing circuit in the electronics world. They can be controlled either using voltage or current. The VCOs are designed using Ring Oscillators or LC Tank oscillators. The high performance oscillators are part of the LC Tank domain while the low frequency oscillators are done using Ring Oscillator.
The design of a high performance voltage controlled oscillator, VCO, is not is not a trivial task. Consideration of the circuit, components used and the layout all play a role in determining the performance. This requires sound theoretical design, followed by careful choice of all the components and then a good custom layout. Even with circuit simulation, it may take a couple of iterations of the VCO layout.
To make a VCO, the oscillator needs to be tuned by a voltage. This can be achieved by making the variable capacitor from varactor diodes. The tune voltage for the VCO can then be applied to the varactors.