Exercise 8 Reactive Properties Of Copper Cable

Exercise 8 Reactive Properties Of Copper Cable

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Objectives for Exercise 8 At the end of this Exercise, you will be able to: Explain the difference between resistive and reactive cable properties. ■ State the two types of reactive cable properties. ■ Describe the physical and electrical characteristics of capacitors. ■

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Objectives (Continued) Explain the effects of cable capacitance on pulse waveforms. ■ Describe how the reduction of capacitance and resistance improve pulse waveforms. ■ Define “induction” as it applies to data communications. ■ List the two ways to reduce the effects of induction. ■

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Electrical Properties Of Copper Cabling ■

As you learned in the previous exercise, cable performance relies on the electrical properties of the cable.

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Specifically you learned that resistance attenuates signal strength.

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Types Of Electrical Properties ■

In electrical terms, there are two types of component properties: – Resistive properties, which offer a constant opposition to current flow, and – Reactive properties, which offer varying opposition to current flow in response to changes in voltage.

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Reactive Properties Of Copper Cable ■

Capacitance – A property that oppose changes in voltage.

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Impedance – The total opposition to current flow in a circuit containing resistance and reactance.

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Induction – The way current in one conductor affects other nearby conductors.

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Capacitance ■

Capacitance stores electrical energy by means of an electrostatic field.

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The electrical component that performs this function is called a capacitor.

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Capacitors oppose any change in voltage.

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Capacitors ■

Although a copper cable does not contain physical capacitors, a certain amount of capacitance exists between two conductors in close proximity. This reacts the same as a small physical capacitor.

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For that reason, let’s start by first examining how a capacitor works.

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Physical Capacitors ■

The principal parts of a capacitor are two metal plates, separated by a non-conductive material called a dielectric. In a cable, these plates would be the two conductors.

Lead Dielectric

Plates Lead

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Capacitance of Wire Pairs and Coaxial Cable

Air Dielectric

Teflon Dielectric

Shield

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Symbology ■

The schematic symbol for a capacitor is shown below.

=

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What Determines the Amount of Capacitance? ■

Capacitance is: – Directly proportional to the area of the plates – Inversely proportional to the distance between the plates – Effected by the material of the dielectric

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The unit of measure for capacitance is the farad. – This is a very large unit. Most capacitors are in the microfarad or picofarad range.

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Capacitance Charging When the switch is moved to the battery position, the capacitor would immediately charge to the value of the battery voltage were it not for the circuit resistance. Charging Current Resistance

+

+

__ __ Resistance

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Charging Rate ■

Because of the circuit resistance, the capacitor will charge at an exponential rate, as shown in this chart. Notice it takes 5 time constants to fully charge - but what is a “time constant?”

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Time Constants ■

One time constant is equal to the circuit’s resistance multiplied by its capacitance. – In the first time constant, the capacitor will charge to 63% of the applied voltage. – In each additional time constant, it will charge to 63% of the remaining voltage difference.

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After 5 time constants, it is effectively charged to the total applied voltage.

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Capacitance Discharging ■ When voltage is removed, the

capacitor will discharge at the same exponential rate that it charged. Discharge Current Resistance

+ Resistance

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Effect of Capacitance on Waveform ■

Since capacitance opposes sudden changes in voltage because of charge time, the leading and trailing edges lose their sharpness and become rounded. V O L T A G E

Ideal Waveform

Effects Of Capacitance

TIME

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Effect of Capacitance vs Pulse Width ■

As data rates increase, individual pulse widths decrease. – As you can see, this greatly degrades the pulses.

V O L T A G E TIME

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Effect of RC Values on Charge Rates ■

As cable capacitance and/or resistance is reduced, thus reducing the time constant, response to sudden voltage changes improves.

Long TC

Medium TC

Short TC

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Cable Impedance We touched on cable impedance earlier in Exercise 2. ■ There we said that impedance: ■

– Opposed changes in current or voltage. – Is the vector sum of resistance and reactance. ■

Let’s now expand upon that a little.

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Determining Impedance In a resistive circuit, current and voltage are in-phase. That is, as voltage rises, current rises at the same time. ■ In a capacitive circuit, current changes lead voltage changes by 90 degrees. ■ Impedance is the total opposition to current flow in a reactive circuit. ■

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Plotting Impedance DC resistance (R) is plotted at 0 degrees. ■ AC resistance (XC) is plotted at 270 degrees. ■

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Impedance (Z) is the vector sum of R and Xc

90

R&I

180

0

Z2 = R2 +XC2

XC Z

270

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Capacitive Reactance (XC) ■

We will not attempt to compute capacitive reactance since it is not something you will be required to do in cable installation or maintenance. However, as you can see by the equation, it is dependent on frequency. XC = 1/2 x (pi) x F x C Where Pi = 3.14, F = frequency in Hz, and C = capacitance in farads

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What Is Induction? As current flows through a conductor, an electromagnet field builds up around it. ■ If the current flow changes, the lines of force move, either expanding or contracting. ■ Whenever moving lines of force cut through another conductor, current is induced to flow in that conductor. ■

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Concerns About Induction in Copper Cabling ■

Noise – Stray radiation can be induced into data cables from electric motors and other equipment, resulting in random signals that mask pulses.

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Cross-Talk – Data pulses can be induced into nearby cables.

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Security – Sensitive data can be radiated from data cables and captured by unauthorized persons.

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Limiting the Effects of Induced Radiation ■

Shielding – Grounded shields keep data in and stray radiation out.

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Cancellation – In paired wiring, both wires carry the same signal but of opposite phase. Magnet fields of opposite phase tend to cancel each other.

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Grounded Shielding ■

In coaxial cables with a grounded shield, both externally and internally generated electromagnet fields are shunted to ground. Shield

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Field Cancellation ■

Since this signal is on both wires, but 180 degrees out of phase, the generated fields tend to cancel each other. – This effect is enhanced by twisting the wire pair.

Source

Load

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Now It’s Your Turn

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