Rise in voltage at the end of a mismatched transmission line - Page 1
Forum section: electronics / beginners

author: rgujju
author_rank: Newbie
timestamp: November 11, 2020, 01:54:37 pm
content: Hello guys,I saw this video by w2aew and wanted to know why there is a rise in voltage at the termination end when there is a mismatched impedance.1) Is it because the reflected wave peak and transmitted wave peak are in phase at that point?2) Does it have anything to do with Ferranti effect?3) When does this increase in impedance occur, when the line is ended in high impedance or low impedance or any mismatched impedance?4) When it is an open or short termination would the voltage be double at the termination?

author: RoGeorge
author_rank: Super Contributor
timestamp: November 11, 2020, 02:24:40 pm
content: It's easier to understand all the effects when thinking about a step shaped signal.Does this clarified your questions?

author: Bud
author_rank: Super Contributor
timestamp: November 11, 2020, 04:43:05 pm
content: Quote from: rgujju on November 11, 2020, 01:54:37 pm1- yes, look up standing wave effect in transmission lines for different terminating impedance2- never heard of that gentlemen's name in relations to transmission lines reflection coefficient. Ferranti effect seems to apply to very long power lines, whereas reflection coefficient and standing wave applicable to any length3- question does not make sense, "increase in impedance" and "low impedance" are mutually exclusive4- obviously if you terminate with a short, the voltage is going to be zero. The rise will be observed when the termination impedance is higher than TL impedance , i recommend you look up transmission line equations for open, equal and zero terminating impedance.

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2020, 05:44:07 pm
content: Often, it is convenient to “series terminate” a transmission line: a series resistor is added between the voltage source and the transmission line, where the resistance increases the output impedance to equal the characteristic impedance of the line. Of course, this assumes a low-impedance source. The receive end of the line must present a high impedance (including shunt capacitance) compared with Z0. If you launch a short pulse down a long cable and observe both ends, the pulse at the source end starts at half the source voltage, but the positive reflection at the high impedance load increases it back to the original voltage. The reflection then propagates back to the source where it is absorbed by the output impedance. If there is an error in the output impedance, a second smaller pulse may bounce back.

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2020, 12:53:53 am
content: Quote from: rgujju on November 11, 2020, 01:54:37 pmYes.QuoteYes, it is the same thing, although the Ferranti effect was observed with sine waves and not pulses.QuoteWhen the end of the transmission line is open, then the reflection doubles the input voltage measured where the input termination meets the transmission line. If series termination is used at the input, then the initial voltage is halved and the doubling of the reflection restores the original voltage. In a point-to-point application, the transmission line only needs to be terminated at one end.

author: AndyC_772
author_rank: Super Contributor
timestamp: November 13, 2020, 07:46:58 am
content: https://cawteengineering.com/design-focus-high-speed-digital-design-and-termination/