## Voltage controlled oscillators

#### A voltage controlled oscillator is a device which produces a sinusoidal (sometimes square wave) output whose frequency is a function of the applied control voltage.

#### Below is a plot for a typical VCO operating with a frequency near 10 MHz, showing output frequency against control voltage. Ideally, the VCO would give a frequency change that is linearly proportional to the applied voltage as shown by the straight line. In practice, however, most practical VCOs have a response which is characteristically 'S' shaped with a near-linear central portion and greater deviation at the extremes of the frequency range.

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#### The frequency varying element of a high frequency VCO is usually a 'varactor diode', whose capacitance varies as a function of applied voltage. This is then used as part of an L-C tuned circuit in the feedback path of an amplifier, thus forming an tuneable (voltage controlled) oscillator. Often the inductance is realized using a quartz crystal which has a very high Q and hence produces an accurate and repeatable frequency of oscillation. These devices are usually called a voltage controlled crystal oscillator or VCXO.