📚 Chapter 4: Guided Transmission Media and Transmission Cables

🎯 Learning Objectives

By the end of this lesson, students should be able to:

Explain how metallic cables distort signals through LRC effects, attenuation, and EMI.
Compare STP, UTP, and coaxial cables using real manufacturer specifications.
Describe how reflections occur and why termination matters.
Use an oscilloscope and pulse generator as a manual TDR to measure cable length and impedance.
Interpret reflection signatures for open, short, and mismatched loads.

1️⃣ Signal Distortion in Metallic Cables

1.1 LRC Model and Pulse Deformation

Metallic cables behave like distributed resistance, inductance, and capacitance.
These properties slow rise/fall times and deform pulses:
The LRC properties determine how fast the voltage can rise or fall… this results in the square input signal deforming slightly.
As frequency increases (pulse width decreases), distortion increases: The higher the frequency is, the more deformed the signal becomes because of the LRC properties.

    graph LR
    A[Input Square Pulse] --> B((Cable LRC Model))
    B --> C[Output: Rounded Pulse]

1.2 Attenuation and Bandwidth

Cables act as low‑pass filters: Copper wire cable acts as a low pass filter where low frequencies pass through, but higher frequencies are attenuated.

Bandwidth is defined as the highest frequency with <3 dB attenuation.

1.3 Electromagnetic Interference

EMI (motors, power lines, fluorescent lights)
Crosstalk between adjacent conductors
Signal reflection from impedance mismatches Signal reflection occurs if a signal traveling down the cable hits an open circuit… Some of the energy is reflected or echoed down the wire

2️⃣ Cable Types

2.1 Shielded Twisted Pair (STP)

Shield + drain wire
Used in noisy industrial environments
Must be grounded at one end only
Typical impedance: 100 Ω The shield reduces the amount of external electromagnetic interference… To be effective, the shield must be properly terminated at only one point.

Why a shield exists in the first place

A shield’s job is simple:

Intercept external electromagnetic fields
Conduct the induced noise safely to ground
Prevent that noise from appearing on the inner signal conductors

To do this, the shield must be at a stable reference potential—normally earth or chassis ground.

What goes wrong if you ground both ends?

1. Ground loops form
If both ends of the shield are tied to ground, and those two grounds are at slightly different voltages (which is extremely common), a loop current flows through the shield.
Even a few millivolts of difference across a long cable produces measurable current.
That current flows through the shield, creating its own magnetic field.
That magnetic field couples directly into the signal conductors. This adds noise instead of removing it.

    graph LR
    A(Ground A) -- small voltage difference --> B(Ground B)
    B --> C((Shield Loop Current))
    C --> D[Induced Noise in Signal Pair]

1. The shield becomes part of the signal return path

If grounded at both ends, the shield can unintentionally carry:

Return currents from nearby equipment
Fault currents
High‑frequency noise currents

This makes the shield behave like a noisy conductor, not a protective barrier.

1. The shield becomes an antenna

A loop of conductive material with two ground points forms a large loop area. That loop:

Picks up magnetic fields (like a receiving antenna)
Radiates noise (like a transmitting antenna)

Either way, it worsens EMI performance.

1. Reflection and impedance problems

For high‑frequency signals, grounding both ends creates:

Multiple return paths
Unpredictable impedance
Standing waves or reflections

This is especially problematic in industrial networks like PROFIBUS or FOUNDATION Fieldbus, where the shield must behave as a clean reference.

Why grounding at one end solves the problem

Grounding at one end:

Gives the shield a stable reference
Provides a safe path for induced noise
Prevents loop currents
Keeps the shield from carrying unintended return currents
Maintains predictable impedance

This is why the course notes emphasize: To be effective, the shield must be properly terminated, at only one point…

When would you ground both ends?

There are exceptions, but they require controlled conditions:

High‑frequency RF coaxial systems (where the shield is part of the transmission line)
Buildings with equipotential bonding systems
Special EMC‑designed installations

These are not typical for industrial fieldbus or twisted‑pair data cables.

2.2 Unshielded Twisted Pair (UTP)

Relies on twisting to reject common‑mode noise
Categories (Cat 3 → Cat 6) with increasing bandwidth
Cat 5e supports 1000 Mbps Ethernet (1G)

2.3 Coaxial Cable

Inner conductor + dielectric + braided shield
Higher bandwidth than twisted pair
Typical impedance: 75 Ω

3️⃣ Lab 04 — Transmission Cable (Manual TDR)

3.1 Concept

Use a pulse generator + oscilloscope to observe reflections and infer:

Cable length
Characteristic impedance
Whether the far end is open, shorted, or terminated
Impedance mismatches between segments The time delay between the incident pulse and the reflected pulse can be used to calculate the length of the cable, and the amplitude of the reflected pulse can indicate whether the far end is open, shorted, or properly terminated.

Why impedance matters in transmission lines

A transmission line (like the BL/BL‑WT pair in Lab 04) has a characteristic impedance $Z_0$ determined by its geometry and materials.
This is not a resistor; it is the ratio of voltage to current for a wave traveling down the line.
When a pulse reaches the far end, one of two things happens:
- If the load impedance $Z_L=Z_0$, the wave is fully absorbed.
- If $Z_L\neq Z_0$, part of the wave reflects back toward the source.

How reflections relate to power transfer

The classic maximum power transfer theorem says:

\[\mathrm{Maximum\ power\ is\ delivered\ when\ }R_L=R_S.\]

In transmission lines, the same idea applies locally at the end of the cable:

The cable behaves like a source with internal impedance $Z_0$.
The load must match $Z_0$ to absorb all the energy.
If the load does not match, the unused energy reflects back. So the “maximum power transfer” idea is still true, but the reason we care is different:
In DC circuits → we want maximum power delivered to a load.
In transmission lines → we want zero reflection, not maximum power.

The cable is not trying to deliver power; it is trying to deliver information. But the physics is the same: matching prevents leftover energy from bouncing back.

Why the cable’s characteristic impedance is constant

People often wonder why the cable has a fixed impedance even though it’s made of copper wire. The answer:

$Z_0$ is determined by geometry (spacing, diameter, dielectric), not DC resistance.
It is the impedance “seen” by a traveling wave, not by a DC current.

This is why Cat 5e, coax, and Fieldbus cables all specify impedance:

UTP/STP: ~100 Ω
Coax: 50 Ω or 75 Ω
PROFIBUS PA: 100 Ω

These values ensure predictable behavior at high frequencies.

3.2 Practice Workflow

Generate a 100 kHz square wave.
Inject into BL/BL‑WT pair. (blue-blue-white twisted pair)
Observe the initial pulse and the reflected step.
Measure Δt between edges.
Compute cable length:

$v\approx 0.70c,\quad L=\frac{v\cdot \Delta t}{2}$

Short the far end → reflection inverts.
Insert potentiometer → adjust until reflection disappears → Z₀ found.

3.3 Reflection

    sequenceDiagram
    participant Src
    participant Cable
    Src->>Cable: Incident Pulse
    Cable-->>Src: Reflection (Open = +, Short = -)

4️⃣ Assignment 4 Prep - Why do copper cables behave as low‑pass filters?

Why does STP require grounding at only one point?
Why does coax support higher bandwidth than UTP?
What are microbend losses in fiber?
Microbend losses are caused by the physical bending of the cable during installation…, this can cause the light to leak out of the core and be lost.