Oscillators

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 a 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, metal detectors and even
stun guns.

To understand how electronic oscillators work, it is helpful
to look at examples from the physical world. In this post,
you'll learn the basic idea behind oscillators and how
they're used in electronics.


Oscillation Basics

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. An electronic oscillator works on the
same principle.

Oscillator Circuits

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. 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 circuit in the picture:

If you charge up the capacitor with a battery and then insert
the inductor into the circuit, here's what will happen:

* The capacitor will start to discharge through the
inductor. As it does, the inductor will create a magnetic
field.
* 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.

­ This 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
capacitor.

Resonators

In a simple crystal radio , a capacitor/inductor oscillator
acts as the tuner for the radio. It is connected to
an antenna and ground like the picture.

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 ignored.

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!