A Controlled Oscillator is an oscillator device where output frequency is proportional to its input. Based on the type of signal applied at input there are different types of controlled oscillators available such as TCXO (Temperature Compensated Crystal Oscillators), OCXO (Oven Controlled Crystal Oscillators), VCXO (Voltage Controlled Crystal Oscillators), DCXO (Digitally Compensated Crystal Oscillators), and others.