Understanding Reflections in High Speed Digital Designs

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Discussion Overview

The discussion focuses on reflections in high-speed digital designs, particularly regarding the role of terminating resistors in matching impedance between driver ICs and transmission lines on PCBs. It explores concepts from transmission line theory, the effects of reflections on signal integrity, and the conditions under which reflections occur.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants argue that the purpose of the terminating resistor is to match the impedance of the transmission lines on the PCB rather than to decrease the current drive of the IC.
  • Others suggest teaching transmission line theory to clarify the role of impedance matching.
  • Concerns are raised about the effects of echoes on pulse shape and inter-symbol interference, with references to the speed of signals on transmission lines.
  • There is a disagreement regarding the speed of signals on transmission lines, with one participant asserting it is 0.4m per nanosecond and another suggesting it is actually 0.2m per nanosecond.
  • One participant questions why reflections only occur when the transmission line approaches the signal wavelength, expressing difficulty in explaining this phenomenon.
  • Another participant counters that reflections can occur even when transmission lines are shorter than a wavelength, noting that the perception of echoes changes based on the timing of reflections relative to the signal cycle.
  • A reference is made to the behavior of a quarter-wave transmission line transformer and the conditions under which reflections occur due to mismatched terminations.

Areas of Agreement / Disagreement

Participants express differing views on the purpose of terminating resistors and the conditions under which reflections occur. There is no consensus on the speed of signals on transmission lines, and the discussion remains unresolved regarding the explanation of reflections in relation to transmission line length and wavelength.

Contextual Notes

Some limitations include the dependence on definitions of impedance matching and the complexity of transmission line behavior, which may not be fully addressed in the discussion.

likephysics
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In high speed digital designs, there is a series terminating resistor to match the output impedance of the driver IC with the transmission line on the PCB. Some think the purpose of this resistor is to decrease the drive of the IC.
How do I convince them its not to decrease the (current) drive but to match the impedance?
 
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Teach them transmission line theory.

Actually it's not there to match the impedance of the output of the driving IC but the impedance of the lines on the board.
 
I guess you could tell them about the effects of echos on pulse shape and inter-symbol interference. Also about the 0.4m per nanosecond speed of signals on transmissions lines.
 
More like 0.2m per nanosecond, actually.
 
sophiecentaur said:
I guess you could tell them about the effects of echos on pulse shape and inter-symbol interference. Also about the 0.4m per nanosecond speed of signals on transmissions lines.

This would be too much to digest.
 
The question I am struggling with is - why do reflection only occur when the tx line approaches signal wavelength.
Why doesn't this occur at wave length >> tx line.
I'm not able to come up with a good explanation.
 
Reflections do occur when transmission lines are shorter than a wavelength it's just that when the time of reflection is less than a cycle, the echo isn't perceived so much as an echo but as a change in impedance of the load. For instance, if a transmission line is 1/4 wavelength and is left unterminated (open) the signal will travel 1/4 wavelength, bounce off the open, and reflect back to the source 180 deg out of phase with the transmitted signal. To the source it appears the load is a short not an open.

In testing RF amplifiers we frequently used stretch lines (or trombone lines) with a short on the end to determine if there were instabilities at any reflection phase angles.
 
Last edited:
likephysics said:
The question I am struggling with is - why do reflection only occur when the tx line approaches signal wavelength.
Why doesn't this occur at wave length >> tx line.
I'm not able to come up with a good explanation.
Understand a quarter wave (transmission line) transformer at

http://en.wikipedia.org/wiki/Quarter-wave_impedance_transformer

then you will see that reflections always occur when the termination is mismatched.

Bob S
 

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