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.

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.