Lab 1:Pulse Propagation and Dispersion

NAME NAME NAME

Introduction:

In this experiment you will observe reflection and transmission of incident pulses as they propagate down a coaxial transmission line.

You will observe the effects of matched and mismatched termination impedances on the amplitude and sign of the reflected pulses. By measuring the time between the arrival of the incident and reflected pulses, you will calculate the length of the line. By varying the pulse width you will observe the superposition of the incident and reflected pulses.

Control Settings:

Scope / Pulse Generator
Vertical Scale: 1.0 V/div / Frequency: 50 kHz
Horizontal Scale: 0.2µs/div / Pulse Width: 0.2 µs
Amplitude: 4 V

Procedure:

  1. Connect the pulse generator to both the oscilloscope and the transmission line. Leave the end of the transmission line open. Use the control settings as a reference to display the incident and the reflected pulseson the oscilloscope. (This should be set up for you but you should move the trace horizontally so that the initial pulse lines up with a vertical axis trace on the oscilloscope.) Carefully measure the time delay between the incident and the reflected pulses. You will use this information to calculate the length of the line.

The time delay between the incident pulse and the first reflected pulse:

s.

Look up the cable characteristics “on line”. You will see the type of cable stamped on the cable. The website should give, the characteristic impedance and L, the inductance per unit length. From these calculate C and thuscalculate , the phase speed on the line.

= ______Ohms

L = ______Henries/m

C = ______Farads/m

Thus the phase velocityof the pulses is= .

The length of the line: m.

Using the expression in the book for C for a coaxial line, taking the magnetic permeability as the vacuum value calculate the ratio of the outer conductor radius to the inner conductor radius and the relative dielectric coefficient of the insulator between the inner and outer conductors.

= ______

(Outer radius) /(Inner radius) =______

  1. Using the control settings as a guide, display the incident and reflected pulses on the scope. Terminate the line with the various terminations provided. Below note the type of termination used and carefully draw the incident and reflected (if any) pulses. Please label Voltage/Div and Second/Div on your graph as you set up on the oscilloscope.

Termination: Short

Theoretical reflection coefficient at the load is: ______

Measured reflection coefficient at the load is: ______

What causes the discrepancy between the measured and the calculated reflection coefficient?

Termination: 50Ω

Did you see a reflected pulse? Why?

Termination: 10Ω

Theoretical reflection coefficient is: = .

Measured reflection coefficient is: = .

Why does the reflected pulse look different from the incident pulse?

Termination: 100Ω

Measured reflection coefficient is: .

Theoretical reflection coefficient is: .

Termination: Open

With an open termination on the line vary the width of the incident pulse and observe the effect of this change on the waveform displayed on the scope. Draw the waveform below and explain briefly what has happened.

Let a transmission line having characteristic impedance[Ω], attenuation constant [m-1] and length l [m] be terminated in a load resistor[Ω].

Let [V] be the amplitude (magnitude) of the incident pulse at the generator, and [V] be the amplitude (magnitude) of the reflected pulse at the generator. The reflection coefficient at the load is. Therefore, the reflection coefficient magnitude of the load measured at the generator is:

From your measured in the cases of short, 10Ω, and 100Ωloads, use the above equation to calculate the attenuation constantm-1.

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