Transmission line reflection effects and solutions - Page 1
Forum section: electronics / projects

author: PauloConstantino
author_rank: Regular Contributor
timestamp: June 12, 2017, 01:05:32 pm
content: I have a question that has come to mind lately and I'd like to both speak out my reasoning to see if it's correct, and then ask a few questions. I will be using the 74HC IC's as an example.As I understand it, if the rising time of the square wave signal is shorter than the time it takes the signal to travel forth and back again along the line, then signal reflections may occur at both ends of the conductor if the impedance of the transmission line is not matched with a proper termination. This means that if the rising time of a signal is faster than the electrical length of the conductor for a round trip, then in looser terms, the signal starts to rise at the driver, and then starts travelling down the conductor, when it reaches the other end, if the input impedance of the receiver is higher than the transmission line impedance, then that voltage divider will add to the reflected voltage, which will travel back to the driver. The reflected voltage will be the voltage at the driver, plus the difference in voltage between the transmission line and the receiver (The voltage divider effect).So the point of matching the impedance of the transmission line, for example adding a resistor in series right after the driver for instance, is to increase the impedance of the transmission line + driver output impedance to match that of the receiver input impedance, so that the voltage divider between the receiver and the transmission line + driver equally distributes the voltages, so that one isn't higher than the other and hence no reflection will occur.I have a feeling that I heard it bring said before that the terminating resistor should match the transmission line impedance, but to my mind, it only makes sense for the resistor to match Rin - Zo - Rout, where Zo is the impedance of the line, Rin is the input impedance of the receiver, Rout is the output impedance of the driver.Since the magnitude of the reflection at the receiver is given by Vout + Vout*(Rin-Zo)/(Rin+Zo), and this is clearly Vout + the difference in voltage between the line and input, so as I understand it, we should add the resistor so that Rin - Zo becomes zero, and in a sense we are adding to the impedance of the line, so that term becomes zero.Is that correct ?I have a second question....... If rather than adding a resistor to terminate the line, is it possible to cut the transmission line in half, and add a buffer in between, so that the length of the line is actually halved? if the line was too long before, and by cutting it in half by adding a buffer in between, which isolates both lines, makes them independent, then this should remove reflections right? Would this be true? Because it seems to me like a good solution, but maybe I might be missing something. Since the gate of a CMOS IC is a capacitor, that isolates one line from the other, and hence reflections can't travel back into the first line. So by adding 2 buffers you would reduce it even more and so on.Thanks everyone.

author: PauloConstantino
author_rank: Regular Contributor
timestamp: June 13, 2017, 09:00:37 pm
content: Sorry for the wait. I will reply soon. Thanks in advance!