Wire Propagation Effects

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Wire Propagation Effects

• Propagation Effects
• Signal changes as it travels
• If change is too great, receiver may not be able
  to recognize it

Original Signal
Final Signal
Distance
2
Wire Propagation Effects Attenuation

• Attenuation Signal Gets Weaker as it Propagates
• May become too weak for receiver to recognize

Signal Strength
Distance
3
Wire Propagation Effects Distortion

• Distortion Signal changes shape as it propagates
• Adjacent bits may overlap
• May make recognition impossible for receiver

Distance
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Wire Propagation Effects Noise

• Noise Thermal Energy in Wire Adds to Signal
• Noise floor is average noise energy
• Random energy, so noise spikes sometimes occur

Spike
Signal
Signal Strength
Error
Noise
Noise Floor
Time
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Wire Propagation Effects

• Noise and Attenuation
• As signal attenuates, gets closer to noise floor
• So noise errors increase with distance, even if
  the average noise level is constant

Signal Strength
Signal
Noise Floor
Distance
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Wire Propagation Effects SNR

• Want a high Signal-to-Noise Ratio (SNR)
• Signal strength divided by average noise strength
• As SNR falls, errors increase

Signal Strength
Signal
SNR
Noise Floor
Distance
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Wire Propagation Effects Noise Speed

• Noise and Speed
• As speed increases, each bit is briefer
• Noise fluctuations do not average out as much
• So noise errors increase as speed increases

OK
Error
One Bit
One Bit
Noise Spike
Noise Spike
Low Speed (Long Duration)
High Speed (Short Duration)
Average Noise During Bit
Average Noise During Bit
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Wire Propagation Effects Interference

• Interference
• Energy from outside the wire (nearby motors,
  other wires, etc.)
• Adds to signal, like noise
• Often intermittent (comes and goes), so hard to
  diagnose

Signal
Signal Strength
Interference
Time
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Wire Propagation Effects Cross-Talk Interference

• Cross-Talk Interference
• Often, multiple wires in a bundle
• Each radiates some of its signal
• Causes cross-talk interference in nearby wires

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Wire Propagation EffectsCross Talk

• Wire Usually is Twisted
• Usually, several twists per inch
• Interference adds to signal over half twist,
  subtracts over other half
• Roughly cancels out
• Simple but effective

-

Interference
Signal
Single Twist
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Wire Propagation EffectsCross Talk

• Terminal Cross-Talk Interference
• Wire must be untwisted at ends to fit into
  connectors
• So cross-talk interference is high at termination
• Problems severe if untwist more than about 1.25
  cm (1/2 inch)
• Usually the biggest propagation effect

Terminal Cross Talk
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Practical Issues in Propagation Effects

• Distance limits in standards prevent serious
  propagation effects
• For instance, usually 100 meters maximum for
  ordinary copper wire
• If stay within limits, usually no serious
  problems
• Problems usually occur at connectors
• Crossed wires
• Poor connections
• Cross-talk interference

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