Transmission line matching

In summary: Senior Engineer at Antennas Direct, responds:Reflections always occur when the termination is mismatched. This is because reflections occur when the impedance of the load (the thing being reflected) is not equal to the impedance of the transmission line. The mismatch creates an echo, which is just a distortion of the original signal.
  • #1
likephysics
636
2
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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  • #2
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.
 
  • #3
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.
 
  • #4
More like 0.2m per nanosecond, actually.
 
  • #5
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.
 
  • #6
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.
 
  • #7
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.
 
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  • #8
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
 

What is transmission line matching?

Transmission line matching is the process of optimizing the impedance of a transmission line to match the impedance of the source and load in order to minimize reflections and maximize power transfer.

Why is transmission line matching important?

Transmission line matching is important because it ensures efficient and reliable transfer of signals or power between a source and load. If the impedance is not matched, it can result in signal distortion, loss of power, and potential damage to the components.

What are the methods for achieving transmission line matching?

There are several methods for achieving transmission line matching, including using impedance matching networks, adjusting the length or width of the transmission line, and using reactive elements such as inductors and capacitors.

What factors affect transmission line matching?

The factors that can affect transmission line matching include the characteristic impedance of the transmission line, the frequency of the signal being transmitted, the length and termination of the transmission line, and the impedance of the source and load.

How do you measure the success of transmission line matching?

The success of transmission line matching can be measured by using a network analyzer to measure the reflection coefficient and return loss of the transmission line. A lower reflection coefficient and higher return loss indicate better matching.

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