A Level Physics - Telecommunications - A Basic Introduction Sound waves Microphones Receivers and transmitters Amplitude modulation (am) Frequency modulation (fm) MICROWAVES Satellite Communication Optical fibers Attenuation The Public Switched Telephone Network

2. SOUND WAVES
Sound waves are longitudinal waves.
Longitudinal waves are waves in which the motion of the individual particles of the
medium is in a direction that is parallel to the direction of energy transport.
A longitudinal wave can be created in a slinky if the slinky is stretched out in a
horizontal direction and the first coils of the slinky are vibrated horizontally. In
such a case, each individual coil of the medium is set into vibrational motion in
directions parallel to the direction that the energy is transported.

3. MICROPHONES
Microphones are a type of transducer - a device which converts energy from one form
to another. Microphones convert acoustical energy (sound waves) into electrical
energy (the audio signal).
Different types of microphone have different ways of converting energy but they all
share one thing in common: The diaphragm. This is a thin piece of material (such
as paper, plastic or aluminum) which vibrates when it is struck by sound waves. In a
typical hand-held mic like the one below, the diaphragm is located in the head of the
microphone.
When the diaphragm vibrates, it causes other components in the microphone to vibrate.
These vibrations are converted into an electrical current which becomes the audio
signal.
Note: At the other end of the audio chain, the loudspeaker is also a transducer - it
converts the electrical energy back into acoustical energy.

4. RECEIVERS AND TRANSMITTERS
The Transmitter:
The information to be transmitted enters the transmitter (microphones are often
the way this is done). Information is amplified and mixed with a radio frequency
signal (carrier wave) generated by the transmitter. This new modulated signal is
amplified and sent to the antenna.
The Receiver:
Radio signals are constantly changing voltages that cycle from positive to negative
thousands, millions or even billions of times per second. This voltage is very, very
small, a few 100 millionths of a volt. It is picked up by the antenna. The signal is
amplified a little bit.
The signal is mixed with an internal signal created by the receiver to lower the
received signal to a much lower frequency. This stepping down might happen a
couple of times. This new lower frequency signal is easier to work with. It then
passed through a filter circuit that isolates the frequencies where the desired
information (voice, music etc.) is. A demodulator circuit "plucks" the information out
of what's left of the radio signal. This Information is put through an audio amplifier
and sent to the speaker(s).

5. AMPLITUDE MODULATION (AM)
An AM signal starts with what's called a carrier wave (i.e. a oscillatory signal like the
sine wave that you've studied in trigonometry) This wave has a set high
frequency and amplitude(i.e., wave height) and is generated electronically by an
oscillator.
A second signal, the modulating signal is produced; let’s say by you singing into a
microphone. The carrier wave and modulating wave are then
combined(electronically multiplied, this is the modulation part, the M in AM) to
produce the modulated carrier wave.
The modulated carrier wave’s amplitude variations match the wave created by your
voice but is converted to a higher frequency of oscillation. The varying
amplitude of the modulated carrier wave carries the information(your voice) to be
transmitted and broadcasted via a transmitter(antenna) for all the world to hear.
This is the radio frequency is what one would tune into in a car radio.

6. AMPLITUDE MODULATION (AM)
Principle of Amplitude Modulation
The amplitude of the signal is basically the vertical lengths of a sinusoidal and the
amplitude can be changed by modulating the audio onto the carrier over time. The
figure below demonstrates this concept.
Just remember the amplitude is modulated.

7. FREQUENCY MODULATION (FM)
Modulate - Alter the amplitude or frequency of (an electromagnetic wave or other
oscillation) in accordance with the variations of a second signal.
Frequency modulation (FM) works by taking a signal, such as an audio signal, and using
it to modulate (see definition above) a higher frequency (Radio Frequency, RF)
carrier.
This modulation causes the RF to shift up and down in frequency. The RF remains
relatively constant in amplitude original signal. Contrast this with Amplitude
Modulation (AM) where the RF is, and its shifting frequency represents the
relatively constant in frequency, but its amplitude represents the original signal.

8. MICROWAVES
Microwaves are a form of electromagnetic radiation with
• Wavelengths ranging from as long as one meter to as short as one millimeter
• frequencies between 0.3 GHz and 300 GHz.
The prefix "micro-" in "microwave" is not meant to suggest a wavelength in the
micrometer range. It indicates that microwaves are "small" compared to waves used in
typical radio broadcasting, in that they have shorter wavelengths.
Microwave technology is extensively used for point-to-point telecommunications
(i.e., non broadcast uses). Microwave signals are similar to radio broadcasting
signals, with the primary difference being that radio waves are longer than a
meter. This means that they are of a higher frequency than radio signals.
Microwave signals have the advantage of being more focused, and more resistant
to interference than radio waves.

9. SATELLITE COMMUNICATION
A satellite is basically a self-contained communications system with the ability to receive
signals from Earth and to retransmit those signals back with the use of a
transponder—an integrated receiver and transmitter of radio signals.
The process begins at an earth station--an installation designed to transmit and receive
signals from a satellite in orbit around the earth. Earth stations send information in
the form of high powered, high frequency (GHz range) signals to satellites which
receive and retransmit the signals back to earth where they are received by other
earth stations in the coverage area of the satellite.
The area which receives a signal of useful strength from the satellite is known as
the satellite's footprint. The transmission system from the earth station to the
satellite is called the uplink, and the system from the satellite to the earth
station is called the downlink.
36,000 Kilometers above the earth.

10. OPTICAL FIBERS
Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter
of a human hair. They are arranged in bundles called optical cables and used to
transmit light signals over long distances.
If you look closely at a single optical fiber, you will see that it has the following parts:
Core - Thin glass center of the fiber where the light travels
Cladding - Outer optical material surrounding the core that reflects the light
back into the core
Buffer coating - Plastic coating that protects the fiber from damage and
moisture
Hundreds or thousands of these optical fibers are arranged in bundles in optical cables.
The bundles are protected by the cable's outer covering, called a jacket.
Optical fibers come in two types:
Single-mode fibers & Multi-mode fibers

11. OPTICAL FIBERS
How Does an Optical Fiber Transmit Light?
Suppose you want to shine a flashlight beam down a long, straight hallway. Just point
the beam straight down the hallway -- light travels in straight lines, so it is no
problem. What if the hallway has a bend in it? You could place a mirror at the bend
to reflect the light beam around the corner. What if the hallway is very winding
with multiple bends? You might line the walls with mirrors and angle the beam so
that it bounces from side-to-side all along the hallway. This is exactly what happens
in an optical fiber.
The light in a fiber-optic cable travels through the core (hallway) by constantly
bouncing from the cladding (mirror-lined walls), a principle called total
internal reflection. Because the cladding does not absorb any light from the
core, the light wave can travel great distances.

12. OPTICAL FIBERS
Fiber-optic relay systems consist of the following:
• Transmitter - Produces and encodes the light signals
• Optical fiber - Conducts the light signals over a distance
• Optical regenerator - May be necessary to boost the light signal (for long
distances)
• Optical receiver - Receives and decodes the light signals

13. ATTENUATION
Reduction of signal strength during transmission. Attenuation is the opposite of
amplification, and is normal when a signal is sent from one point to another. If the
signal attenuates too much, it becomes unintelligible, which is why most
networks require repeaters at regular intervals. Attenuation is measured in
decibels.
All coaxial cables have loss. The term is “Attenuation” and it is measured in Decibels. It's
a logarithmic scale (based on the power to which 10 is raised to equal the factor
expressed in the ratio) where:
1 dB = x 1.26
3 dB = x 2
6 dB = x 4
10 dB = x 10
20 dB = x 100
30 dB = X 1000

14. ATTENUATION

15. THE PUBLIC SWITCHED TELEPHONE NETWORK
The Public Switched Telephone Network (PSTN), also known as Plain Old Telephone
Service (POTS), is the wired phone system over which landline telephone calls are
made. The PSTN relies on circuit switching. To connect one phone to another, the
phone call is routed through numerous switches operating on a
local, regional, national or international level. The connection established between
the two phones is called a circuit.