Ch 1 - P 1 - Balanced Twisted-Pair Cabling

Question 1

What is the main reason for twisting conductor pairs?

Answer 1

To minimize crosstalk and noise by decreasing capacitance unbalance and mutual inductance coupling between pairs.

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Question 2

What does pair-to-pair capacitance unbalance measure?

Answer 2

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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Question 3

What is mutual inductance?

Answer 3

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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Question 4

How is crosstalk within a multipair cable minimized?

Answer 4

By giving each pair a different twist length within a standard range

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Question 5

What type of twist is generally used for voice and low frequency data cables?

Answer 5

A counter clockwise twist length between = 50 mm and =150 mm (1.97 in and 6 in)

Question 6

What is tight twisting?

Answer 6

Creating pair twist lengths that are less than = 12.7 mm ( 0.5 in)

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Question 7

Where is tight twisting commonly used?

Answer 7

Within and between computers and other date processing equipment

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Question 8

Which cable categories employ tight twisting for optimum transmission performance?

Answer 8

Cable 5e, 6, 6A and higher

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Question 9

What term is used to refer to stray electrical radiated from electrical equipment and electronics systems?

Answer 9

Electromagnetic interference (EMI)

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Question 10

At what temperature does attenuation increase in balanced twisted-pair cables?

Answer 10

Above 20 C (68 F)

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Question 11

What temperature reference is cited in cabling standards for all twisted-pair cables?

Answer 11

20 C +/- 3 C (68 F +/- 5.4 F)

Question 12

What is cable shield?

Answer 12

A metallic covering or envelope enclosing an insulated conductor, individual group of conductors within a core, and a cable core.

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Question 13

Name 3 potential benefits of using a cable shield if it is properly terminated, bonded, and grounded (earthed).

Answer 13

1. Reduces the radiated signal from the cable
2. Reduces the effects of electrical hazards
3. Minimizes the effect of external EMI on the conductors within the shielded cable

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Question 14

Name 3 factors that can affect a shield’s effectiveness.

Answer 14

1. Type and thickness of the shield material
2. Number and size of openings in the shield
3. Effectiveness of the bonding connection to ground

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Question 15

How is shield effectiveness determined?

Answer 15

By measuring the surface transfer impedance

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Question 16

What is surface transfer impedance?

Answer 16

The ratio of the conductor-to-shield voltage per unit length to the shield current

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Question 17

Why are solid metal tubes inappropriate for most cable applications?

Answer 17

Because of their rigid nature

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Question 18

Name the 4 primary criteria for selecting cable shields.

Answer 18

1. Nature of the signal to be transmitted
2. Magnitude of the EM fields through which the cable will run
3. EMC regulations
4. Physical environments and specific mechanical requirements
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Question 19

What type of cable shield has poor tensile strength?

Answer 19

Foil (Foil and Braid)

Question 20

Which type of cable shield is the least effective for the radio frequency?

Answer 20

Flexible Conduit

Question 21

What type of cable shield has a poor fatigue life?

Answer 21

Solid conduit

Question 22

Which cable shields have excellent tensile strength?

Answer 22

1. Single layer braid
2. Multiple layer braid
3. Single Conduit

Question 23

What is permeability?

Answer 23

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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Question 24

Name 2 reasons for using a drain wire.

Answer 24

1. To provide an easier means for grounding (earthing) the shield
2. To ensure shield continuity for metallic foil shields
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Question 25

How are drain wires typically applied?

Answer 25

Longitudinally next to the metallic part of the shield for the length of the cable.

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