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.
To understand how electronic oscillators work, it is helpful to
look at examples from the physical world. One of the most common
oscillators you find in common use is the pendulum of a clock. If
you push on a pendulum to start it swinging, it will oscillate back
and forth at some frequency -- it swings back and forth so
many 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, it's 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. Then the pendulum moves toward
the other end of its swing and 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.
Electronic Oscillators
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 form of oscillator by connecting a
capacitor
and an inductor
together. If you read How Capacitors
Work and How
Inductors Work, you will see that both capacitors and inductors
store energy. A capacitor stores energy in the form of an
electrostatic field in the capacitor, while an inductor uses a
magnetic field.
Imagine the following circuit:
If you charge up the capacitor with a battery and then insert the
capacitor 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.
- And so on?
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.
Electronic Oscillators
In a
simple crystal radio (see How Radios Work
for details), a capacitor/inductor oscillator like this 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 the other
frequencies get ignored by the resonator.
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!
| 
.gif)
