A Level Physics - Telecommunications

A LEVEL P

HYSIC

S -

TELE

COMMUNICAT

IONS

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.

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.

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).

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.

AMPLITUDE MODULATION (AM)

Principle of Amplitude ModulationThe 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.

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.

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.

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.

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

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.

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

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

ATTENUATION

THE PUBLIC SWITCHED TELEPHONE NETWORKThe 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.