Principals Of Transmission Flashcards
What is created by twisting or stranding together a number of small-gauge solid conductors?
Stranded conductors
Stranded conductors are used for their flexibility and durability.
List three advantages of solid conductors.
- Less costly
- Less complex termination systems
- Better transmission performance at high frequencies
Solid conductors are preferred in applications requiring high-frequency performance.
List three advantages of stranded conductors.
- More flexible
- Longer flex life
- Less susceptible to damage during crimp termination processes
Stranded conductors are beneficial in applications where movement is required.
What is a composite conductor?
Conductors constructed from nontraditional materials (e.g., metallic resins, graphite)
Composite conductors are often used in low-end audio devices and telephone cords.
List five advantages of composite conductors.
- Highly flexible
- Lightweight
- Inexpensive and easy to produce
- Easily embedded into other materials
- Low coefficient of expansion
The flexibility and lightweight nature make composite conductors popular in specific applications.
List four disadvantages of composite conductors.
- Poor analog transmission characteristics including high attenuation above 4000 Hz
- Extremely poor digital transmission characteristics
- Easily damaged unless encased in rigid material
- Inconsistent quality
These disadvantages limit their use in modern telecommunications.
True or False: Cables with composite conductors are recommended for use with modern telecommunications networks.
False
It is advised to discard and replace such cables with proper structured cabling.
In what types of applications are composite conductors commonly used?
- Telephone receiver cords
- Mounting cords
- Inexpensive headsets
- Low-end audio devices
They are also used to embed audio devices into plastic shells such as helmets.
What is the dielectric constant?
The ratio of the capacitance of an insulated conductor to the capacitance of the same conductor uninsulated in the air
Air is the reference with a dielectric constant of 1.0. A low dielectric constant is desirable and can change with temperature, frequency, and other factors.
What does dielectric strength measure?
The maximum voltage that an insulation can withstand without breakdown
Dielectric strength is recorded in breakdown tests where voltage is increased until insulation fails, expressed in volts per millimeter or volts per mil.
What is the typical dielectric strength range for insulated conductors in telecommunications applications?
Between 7500 and 30,000 V per millimeter (300 and 1200 V per mil)
A high dielectric strength is preferred to withstand voltage stress.
What is the dissipation factor?
The relative power loss in the insulation due to molecular excitement and subsequent kinetic and thermal energy losses
It is of primary concern in high-frequency megahertz (MHz) ranges where signal loss increases due to the structure of the insulating material.
What is the preferred value for the dissipation factor?
A low dissipation factor is preferable
Polar molecules like water absorb energy in an electromagnetic (EM) field, affecting dissipation.
What does insulation resistance (IR) measure?
The insulation’s ability to resist the flow of current through it
IR is typically expressed in megohm kilometer (km) or megohm•1000 feet (ft) and has an inverse relationship with cable length.
Fill in the blank: A high value of dielectric strength is preferred to withstand _______.
[voltage stress]
True or False: A high dielectric constant is generally desirable for insulation materials.
False
Generally, a low dielectric constant is desirable.
What happens to insulation resistance as cable length increases?
Insulation resistance becomes smaller
There is an inverse relationship between insulation resistance and cable length.
What is the main reason for twisting pairs of conductors in balanced twisted-pair cables?
To minimize crosstalk and noise by decreasing capacitance unbalance and mutual inductance coupling between pairs
Twisting also improves physical symmetry and reduces noise coupling from external sources.
What does pair-to-pair capacitance unbalance measure?
The electrical field coupling between two pairs when a differential voltage is applied on one pair and a differential noise voltage is measured on another pair in close proximity.
What does mutual inductance measure?
The magnetic field coupling between two pairs when a differential current is applied on one pair and a differential noise current is measured on another pair in close proximity.
What factors affect both mutual inductance and capacitance unbalance?
The relative length and uniformity of pair twists.
What is the standard twist length range for voice and low-frequency data cables?
Between 50 mm and 150 mm (2 in and 6 in).
What is the minimum twist length difference between adjacent pairs?
At least 13 mm (0.50 in).
What is tight twisting in the context of twisted-pair cables?
An option where pair twist lengths are less than 13 mm (0.50 in), used particularly within and between computers and data processing equipment.
Which cable categories employ tight twisting for optimum transmission performance?
Category 5e, Category 6, and Category 6A cables.
Fill in the blank: The conditions under which crosstalk is measured include both _______ and mutual inductance coupling effects.
[capacitance unbalance]
True or False: Longer twists in twisted-pair cables tend to preserve their shape better than shorter, tighter twists.
False
What is the effect of tight twists on the shape of cables?
Tight twists are less likely to deform.
What is Electromagnetic Interference (EMI)?
Stray electrical energy radiated from electronic equipment and systems, causing distortion or interference to signals in nearby cables or systems.
See Chapter 2: Electromagnetic Compatibility for a detailed discussion of EMI.
What is a concern regarding temperature effects on balanced twisted-pair cables?
Attenuation increase at higher cable temperatures (above 20 °C [68 °F]).
In which locations can high temperatures that affect cable performance be routinely encountered?
- Exterior building walls
- Ceiling spaces, including plenums
- Mechanical rooms
What can cause intermittent failures in local area networks (LANs)?
Solar heating of walls and the cabling inside them.
What must be considered in the premises cabling design process to avoid temperature-related problems?
The attenuation at the highest expected temperature.
What factors cause attenuation to increase with temperature?
- Conductor resistance
- Insulation dielectric constant
- Dissipation factor
What should be consulted regarding cable insertion loss margin?
The manufacturer’s specifications compared with the maximum insertion loss specified in the standard.
