Chapter 6 Report

Chapter 6

Practical 1.2 standing wave and partial reflection

Objectives:

1. To observe partial reflection at a mismatched termination.

2. To observe standing waves on a line with mismatched termination

3. To know that the standing waves are equal to the sum of the incident and reflected

waves.

Equipments:

TLD511 (Transmission line demonstrator), variable phase sine-wave generator (e.g.

Feedback VPG608), 600R resistor, 200R resistor, 1k8 resistor, links

TLD511 Transmission line demonstrator

Procedures:

1. The terminators are removed and the generator, two links and a 600R

terminator are connected as shown in figure below.

2. On the generator, the frequency is set to about 2.5Hz, and the amplitude is

adjusted until the travelling wave on the display is just lighting two sections

up and two down in each column of the display. This is called ±2units of

amplitude.

3. The 600R terminator is removed and a standing wave appears.

4. The 1k8 terminator is substituted. Note that the standing wave is reduced. Its

minimum and maximum values are noted and the values of the reflected pulse

in practical 1.1 are compared.

5. The 1k8 terminator is replaced with one of 200R.

Observations:

When the 600R terminator is substituted with 1k8 terminator, it is found that the

maximum amplitude obtained is +3 units. A reflection coefficient describes either the

amplitude or the intensity of a reflected wave relative to an incident wave, when

wave propagation in a medium containing discontinuities is considered. The reflection

coefficient is closely related to the transmission coefficient.

1k8 terminator is being replaced with 200R terminator, the results obtained have some

similarities. The amplitudes obtained for both are constant positive units when the

‘hold’ button is pressed. However, the results also show some differences. 1k8

terminator only gives results with positive amplitude while 200R can give result with

negative amplitude. Not only that, the maximum amplitude that the 1k8 terminator

can give is +3 units whereas the maximum amplitude that 200R terminator can give is

only +2 units.

Results:

Steps Results Diagram (as shown below)

1. With 600R terminator (frequency of about 2.5Hz)

Amplitude of +2 units 1

2. Remove the 600R terminator Amplitude of +4 units 23. Substitute 1k8 terminator Step input to B: amplitude of +3

unitsStep input to A: -amplitude changes from +2 units to +3 units (press ‘hold’button)-amplitude changes from +1 to 0 and to +1 again (repeatedly)

To B: 3To A: -4(press ‘hold’ button) -5 (‘hold’ button released

4. Replace 1k8 with one of 200R Step input to A: -amplitude from +2 units to +1unit (press ‘hold’button)-amplitude from -1 unit to +1unit(‘hold’button released)Step input to B: -amplitude from constant +1 unit to +1, 0, +1, 0 unit repeatedly(press ‘hold’button)-amplitude with constant of +1unit(‘hold’button released)

To A: -6(press ‘hold’ button) - 7(‘hold’ button released) To B: -8(press ‘hold’ button)

Diagram1 Diagram 2

Diagram 3 Diagram 4

Diagram 5 Diagram 6

Diagram 7 Diagram 8

Discussion:

The maximum amplitude occurs when the incident and reflected waves are both a

maximum at the same time and place. Since the incident wave was ± 2 units of

amplitude, and the reflection coefficient was ½, the reflected wave is ± 1unit. This

added to the incident wave gives a maximum of ± 3 units of amplitude. The reflection

coefficients with 200R and with 1k8 terminators were of opposite sign, or

equivalently they differed by 180’ in phase. Thus the phase of the reflected wave in

the one case must differ by 180’ from the phase of the other reflected wave. To

produce a corresponding change in ‘reflected’ waves supplied by the variable-phase

oscillator, its phase controls must likewise be adjusted by 180’ (e.g. in practice on the

VPG608 the lead/lag switch must be thrown, the phase control set to the difference

between 90’ and its former setting).

Conclusion:

If a transmission line is ‘correctly terminated’, for example in its characteristic

impedance, all the power sent down the line is absorbed at the termination and no

signal returns. Furthermore, if the line is open circuit or short circuit at the

termination, complete reflection of the signal occurs. A wave, travelling in a line of

characteristic impedance Zo and incident on a termination of impedance Zb gives rise

to a reflected wave those amplitude is that of the original wave reaching the

termination multiplied by a reflection coefficient, that has a voltage wave of formula:

¿¿.