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a concept : reflection noise due to impedance mistmatch?

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Hi,

I am confused a concept, pls correct me.

Usually we talk impedance mismatch at high f. but in low f (let's say <10MHz), there still exist reflection noise due to mistance mismatch. let' say load 100ohm, source output impedance 50ohm, and the transmission line in between maybe any value. If we launch a pulse train at input, does the wave shape will be affected by the reflection noise due to this impedance mismatch? If this is the case, then the signal will be degraded, why people never thought low f impedance mismatch problem.
many thanks!
 

That's not really noise, that's interference. You are partially correct in that the reflected wave will distort the pulse train, but there isn't a dependence on frequency there, it's simply dependent on the reflection coefficient. One assumption is that the reflection coefficient is flat across frequency.

Dave
 

I assume You used the term "noise" as "unwanted effect".

Phenomena like pulse distortion , pulse reflection etc. still exist at all frequencies, low and high.
But why usually it's taken in account at high frequencies? Because the lenght of the line is much longer ¼λ .

For such long TX lines , the reflected signal combine it's phase with the forwarding wave (or pulse if You prefere) and the result is the known phenomena called stationary wave by engineers and called interference by scientist.

For lines much shorter than ¼λ the reflected phase is close to the forwardind one.

But if You wrote "noise" thinking to the white, gaussian noise... so the answer will became much longer. Tell me.
 

Thank you guys answering my question!

I guess I get some sense from your answer.
Yes, I mean "unwanted effect". Actually I think of case, like if I send pulse train "1000111....", and I have source output imp. 50Ohm, TX line Z0 100ohm, load 200ohm. then for high f, there would some reflection due to the mismatch, the bit signals at later train would be affected by the previous bit signal reflection. If the TX is short compared with transmitted signal wavelength (low f), the reflection would not play effect because the phase will not change, is that what you mean?
many thanks!
 

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