* Chapter 1 - Principles of Transmission Flashcards
An electrical conductor is
any material that can carry an electrical charge from one point to another
The most common electrical conductors in the ITS industry are
Copper, Copper Covered Steel, High Strength copper alloys and Aluminum
Copper (Conductor Description)
Sets the standard for connectivity of other metals, Annealed copper is used as the reference value (100% conductivity.
Copper-covered (Conductor Description)
Also known as copper clad steel, it combines the conductivity of steel copper with the strangth of steel. It is typically used as a conductor for aeiral, self supporting drop wire.
High-Strength (Conductor Description)
A mixture of copper and other metals to improve certain copper alloy properties and characteristics of copper. Alloys such as cadmium-chronium copper and zirconium copper offer important weight reductions or greater strength. These factors are especially important in aerospace and other applications.
Aluminum (Conductor Description)
Most commonly used in electrical utility distribution lines. It is a malleable ductile light metallic element that has good electrical and thermal conductivity, high reflectivity and resistance to oxidation. It has about 60% conductivity compared with copper and is lighter in weight.
Advantages of Solid Conductors
Less Costly - Less complex Terminations - Better Transmission performance at high frequencies - Less Resistance
Advantages of Stranded Conductors
More Flexible - Longer flex life - Less susceptible to damage during crimping process
Composite Conductor
Term used to describe conductors constructed from nontraditional materials.
Advantages of Composite Conductors
Flexible - Light - Inexpensive and easy to produce - Easily embedded into other materials - Low coefficient of expansion
Fluorinated Ethylene Propylene (FEP)
Teflon, NEOFLON FEP.
Teflon is a trademark of
E.I. du Pont de Nemours & Company.
NEOFLON FEP is a trademark of
Daikin America Inc
Halar is a trademark of
Solvay Solexis
AWG
American Wire Gauge = Generally accepted cable sizing system in North America.
Insulation (also called a dielectric)
Used to isolate the flow of current by preventing direct contact between: Conductors, A conductor and its environment.
Insulation (dielectric) is typically made of
Plastic materials - applied by a variety of methods. Extruded polymers are generally used as insulation
Cables with a lower dielectric constant and disposition factor have
better transmission performance including lower attenuation characteristics and lower capacitance
Dielectrics ______ the electromagnetic coupling between conductors by increasing seperation
reduce
Dielectric Constant
The ratio of the capacitance of an insulated conductor to the capacitance of the same conductor uninsulated in the air. Generally a low dielectric constant is desirable
Dielectric Strength
Measures the maximum voltage that an insulation can withstand without breakdown. A high value for dielectric strength is preferred.
Insulated conductors in telecom applications have a typical dielectric strength between _____ and ____V per millimeter.
7500 and 30,000 V per millimeter (300 and 1200 V per mil)
Dissipation Factor
The relative power loss in the insulation due to molecular excitement and subsequent kinetic and thermal energy losses. A low dissipation factor is peferrable
Insulation Resistance (IR)
The insulation ability to resist the flow of current through it. Typically expressed in megohmkilometer or megohm1,000 feet
As cable length increases, the insulation resistance becomes________
smaller
The main reason for tight twisting pairs of conductors is to
Minimize crosstalk and noise by decreasing capacitance unbalance and mutual inductance coupling between pairs.
A _____ twist between ___ and ___ inches is used for voice and low frequency cables
Counterclockwise, 2in and 6in (.50mm and 150mm)
Adjacent pairs are generally designed to have a twist length difference of at at least__________
12.7mm or .5 inches.
EMI
Electromagnetic Interference
What is EMI?
stray electrical energy radiated from electronic equipment and electronic systems (including cables). EMI can cause distortion or interference to signals in other nearby cables and systems
High Temperatures can be routinely encountered in
Exterior Building Walls
Ceiling spaces, including plenums
Mechanical Rooms
Attenuation increases with temperature because of increased:
Conductor Resistance
Insulation dielectric constant
Dissipation Factor
A temperature coefficient of ______ per degree Celsius is not uncommon for some Category 3 cables
1.5%
What is a shield?
A metallic covering or envelope enclosing the:
- Insulated conductor
- Individual group of conductors within a core
- Cable core
Shields are made of..
Foil or Braided Metal Strands
Usually tinned copper, bare copper, aluminum, or another electrically conductive material
When properly terminated, bonded and grounded (earthed) cable shields can
- Reduce the radiated signal from the cable
- Reduce the effects of electrical hazards
- Minimize the effect of external EMI on the conductors within the shielded cable
The effectiveness of a shield depends on factors such as the..
- Type and thickness of the shield
- Number and size of the openings in the shield
- effectiveness of the bonding connection to the ground
What are the steps to convert analog signals to digital?
- Filtering
- Sampling
- Quantizing/compounding
True or False
The sampling rate must be at least twice the highest frequency component of the analog signal to faithfully reproduce the analog signal as a digital signal.
True
What is a simplex transmission system?
Simplex transmission systems only send signals in one direction
What is the signal strength decibel defined as?
Decibel is defined as:
dB = 10log(P1/P2)
It is the power of signal P1 relative to reference power P2.
3dB indicates a power ratio 2x. So +3dB would double the power.
What the three basic components of a transmission system in telephony?
- source of energy (acoustic energy of speech is converted to electrical signal)
- medium to carry the energy (a balanced twisted pair is commonly used)
- receiving device (transducer in the receiver headset acts a small loudspeaker and converts the electrical signal back to sound energy)
True or False
A conduit is the best possible shield.
True
