Ch 1 - P 1 - Balanced Twisted-Pair Cabling Flashcards
What is the main reason for twisting conductor pairs?
To minimize crosstalk and noise by decreasing capacitance unbalance and mutual inductance coupling between pairs.
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What does pair-to-pair capacitance unbalance measure?
The electric field coupling between two pairs if a differential voltage is applied on one pair and a differential noise voltage is measured on another pair in close proximity
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What is mutual inductance?
A measure of the magnetic field coupling between two pairs if a differential current is applied on one pair and a differential noise current is measured on another pair in close proximity
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How is crosstalk within a multipair cable minimized?
By giving each pair a different twist length within a standard range
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What type of twist is generally used for voice and low frequency data cables?
A counter clockwise twist length between = 50 mm and =150 mm (1.97 in and 6 in)
What is tight twisting?
Creating pair twist lengths that are less than = 12.7 mm ( 0.5 in)
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Where is tight twisting commonly used?
Within and between computers and other date processing equipment
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Which cable categories employ tight twisting for optimum transmission performance?
Cable 5e, 6, 6A and higher
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What term is used to refer to stray electrical radiated from electrical equipment and electronics systems?
Electromagnetic interference (EMI)
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At what temperature does attenuation increase in balanced twisted-pair cables?
Above 20 C (68 F)
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What temperature reference is cited in cabling standards for all twisted-pair cables?
20 C +/- 3 C (68 F +/- 5.4 F)
What is cable shield?
A metallic covering or envelope enclosing an insulated conductor, individual group of conductors within a core, and a cable core.
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Name 3 potential benefits of using a cable shield if it is properly terminated, bonded, and grounded (earthed).
- Reduces the radiated signal from the cable
- Reduces the effects of electrical hazards
- Minimizes the effect of external EMI on the conductors within the shielded cable
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Name 3 factors that can affect a shield’s effectiveness.
- Type and thickness of the shield material
- Number and size of openings in the shield
- Effectiveness of the bonding connection to ground
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How is shield effectiveness determined?
By measuring the surface transfer impedance
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What is surface transfer impedance?
The ratio of the conductor-to-shield voltage per unit length to the shield current
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Why are solid metal tubes inappropriate for most cable applications?
Because of their rigid nature
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Name the 4 primary criteria for selecting cable shields.
- Nature of the signal to be transmitted
- Magnitude of the EM fields through which the cable will run
- EMC regulations
- Physical environments and specific mechanical requirements
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What type of cable shield has poor tensile strength?
Foil (Foil and Braid)
Which type of cable shield is the least effective for the radio frequency?
Flexible Conduit
What type of cable shield has a poor fatigue life?
Solid conduit
Which cable shields have excellent tensile strength?
- Single layer braid
- Multiple layer braid
- Single Conduit
What is permeability?
The property of a magnetic substance that determines the degree in which it modifies the magnetic flux in the region occupied by it in a magnetic field.
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Name 2 reasons for using a drain wire.
- To provide an easier means for grounding (earthing) the shield
- To ensure shield continuity for metallic foil shields
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How are drain wires typically applied?
Longitudinally next to the metallic part of the shield for the length of the cable.
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