Oscillators are important in many different types of
electronic equipment. For example, a quartz
watch uses a quartz oscillator to keep track of what time
it is. An AM radio transmitter uses an oscillator to create
the carrier wave for the station, and an AM radio receiver
uses a special form of oscillator called a resonator to
tune in a station. There are oscillators in computers,
detectors and even stun
To understand how electronic oscillators work, it is
helpful to look at examples from the physical world. In this
edition of HowStuffWorks,
you'll learn the basic idea behind oscillators and how they're
used in electronics.
One of the most commonly used
oscillators is the pendulum of
a clock. If you push on a pendulum to start it swinging, it
will oscillate at some frequency -- it will swing back
and forth a certain number of times per second. The length of
the pendulum is the main thing that controls the frequency.
For something to oscillate, energy needs to move back and
forth between two forms. For example, in a pendulum, energy
moves between potential energy and kinetic
energy. When the pendulum is at one end of its travel, its
energy is all potential energy and it is ready to fall. When
the pendulum is in the middle of its cycle, all of its
potential energy turns into kinetic energy and the pendulum is
moving as fast as it can. As the pendulum moves toward the
other end of its swing, all the kinetic energy turns back into
potential energy. This movement of energy between the two
forms is what causes the oscillation.
Eventually, any physical oscillator stops moving because of
friction. To keep it going, you have to add a little
bit of energy on each cycle. In a pendulum clock, the energy
that keeps the pendulum moving comes from the spring. The
pendulum gets a little push on each stroke to make up for the
energy it loses to friction. See How Pendulum
Clocks Work for details.
An electronic oscillator works on the same principle.
Energy needs to move
back and forth from one form to another for an oscillator to
work. You can make a very simple oscillator by connecting a capacitor
and an inductor
together. If you've read How
Capacitors Work and How Inductors
Work, you know that both capacitors and inductors store
energy. A capacitor stores energy in the form of an
electrostatic field, while an inductor uses a magnetic field.
Imagine the following circuit:
If you charge up the capacitor with a battery
and then insert the inductor into the circuit, here's what
oscillation will continue until the circuit runs out of energy
due to resistance in the wire. It will oscillate at a
frequency that depends on the size of the inductor and the
- The capacitor will start to discharge through the
inductor. As it does, the inductor will create a magnetic
- Once the capacitor discharges, the inductor will try to
keep the current in the circuit moving, so it will charge up
the other plate of the capacitor.
- Once the inductor's field collapses, the capacitor has
been recharged (but with the opposite polarity), so it
discharges again through the inductor.
In a simple crystal radio (see How Radio
Works for details), a capacitor/inductor oscillator acts
as the tuner for the radio. It is connected to an
antenna and ground like this:
Thousands of sine waves from different radio
stations hit the antenna. The capacitor and inductor want to
resonate at one particular frequency. The sine wave that
matches that particular frequency will get amplified by
the resonator, and all of the other frequencies will be
In a radio, either the capacitor or the inductor in the
resonator is adjustable. When you turn the tuner knob
on the radio, you are adjusting, for example, a variable
capacitor. Varying the capacitor changes the resonant
frequency of the resonator and therefore changes the frequency
of the sine wave that the resonator amplifies. This is how you
"tune in" different stations on the radio!
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