Parameters and
properties
of metallic cables
4.lab assignment for course BE4B38PSIA
Lukáš Kulhánek [kulhalu8@fel.cvut.cz]
1
Contents
Intro ........................................................................................................................................................ 3
Used tools ............................................................................................................................................... 3
Measurements ........................................................................................................................................ 4
1. Pulse generator............................................................................................................................ 4
2. Reflection factor at the end of the line ......................................................................................... 4
3. Line length measurement using reflection ................................................................................... 5
4. Line impedance measurement ..................................................................................................... 6
5. Start of line termination ............................................................................................................... 6
Conclusion............................................................................................................................................... 8
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Intro
REFLECTION FACTOR AT THE END OF LINE
The goal of this part is to calculate the reflection factor using the equations given in the materials.
LINE LENGTH MEASUREMENT USING REFLECTION
The goal of this part is to calculate the length of the coaxial cable using oscilloscope and maths
LINE IMPEDANCE MEASUREMENT
The goal of this part is to find the correct termination value for the coaxial cable
START OF LINE TERMINATION
The goal of this part is to observe the signal without termination on the end of the cable
Used tools
Function generator
Oscilloscope
Variable terminator
Multimeter
Coaxial cable
3
Measurements
1. Pulse generator
Figure 1 Frequency generator settings
2. Reflection factor at the end of the line
Given that Z = 50Ω, the values of reflection factor at the end of line can be calculated as follows
𝑅 − 𝑍 0 − 50
𝐹𝑜𝑟 𝑅 = 0Ω: 𝜚𝑏 = = = −1
𝑅 + 𝑍 0 + 50
𝑅 − 𝑍 25 − 50 1
𝐹𝑜𝑟 𝑅 = 25Ω: 𝜚𝑏 = = =−
𝑅 + 𝑍 25 + 50 3
𝑅 − 𝑍 50 − 50
𝐹𝑜𝑟 𝑅 = 50Ω: 𝜚𝑏 = = =0
𝑅 + 𝑍 50 + 50
𝑅 − 𝑍 100 − 50 1
𝐹𝑜𝑟 𝑅 = 100Ω: 𝜚𝑏 = = =
𝑅 + 𝑍 100 + 50 3
𝑅−𝑍 1
𝐹𝑜𝑟 𝑅 = ∞Ω: 𝜚𝑏 = lim = lim = 1
𝑅→∞ 𝑅 + 𝑍 𝑅→∞ 1
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3. Line length measurement using reflection
The line length can be calculated by measuring the delay between the delay between the reflection
happening at the end of the line and it arriving on the start. Given that we know the speed at which the
signal travels, the measured time value can then be used to calculate the length as follows
𝑙 = 𝑣 ∗ 𝑡 = (𝑐 ∗ 0.65) ∗ 𝑡 = (299 792 458 ∗ 0.65) ∗ (8 ∗ 10−7 ) = 155.89 𝑚
Figure 2 Time required for the reflection to arrive at the start of the cable
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4. Line impedance measurement
After some fiddling with the knob on the termination box, the value at which no reflections are
observed seems to be Rz = 51.260Ω
Figure 3 Observed waveform with correct termination value set
5. Start of line termination
Reflection can be seen on the waveform. At the start, the original signal from the generator can be seen,
then the reflection joins in (after it arrives on the start, which is the measuring point). At the end of the
waveform, similar thing can be observed. The generator stops generating the signal, but the reflected
signal is still present in the cable, so it takes some more time for it to disappear.
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Figure 4 The observed waveform
Figure 5 Zoom on the start of the waveform
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Figure 6 Zoom on the end of the waveform
Conclusion
As can be seen from the results, line termination plays an important role in limiting or removing
reflections from the metallic cable line. The reflection phenomenon is probably unwanted in most cases,
however it can be also very useful (for example for measuring the cable length as seen here).
