In terms of a signal reflection, the impedance matching is important only for a transmission line?

Discussion in 'General Electronics Chat' started by Dong-gyu Jang, Feb 1, 2017.

  1. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    115
    4
    Hello.

    To achieve maximum power transfer, the impedance matching is important for both a lumped electrical network or a circuit with a transmission line. However, for a signal reflection, is the matching also important for the lumped network? Let's see the attached figure.

    Transmission line.jpg

    Let's assume ZS ≠ Z0 ≠ ZL. We know that a signal incident on the ZL from Z0 is reflected at their interface. The reflected signal goes toward ZS and is reflected again at the interface of ZS and Z0. So far so good. But, what about when the signal transfers from ZS to Z0 at the first time? Is there also a reflection at the interface and the reflected signal bounces back and forth between ZS and Z0 ?

    This is the question I almost never seen from the textbook or articles so I didn't get it clarified yet. Please help me to solve this.
     
  2. nsaspook

    Expert

    Aug 27, 2009
    6,189
    6,974
    Why does the signal reflection exist? It's exists because the transmission time is not instantaneous from point to point and as the original traveling wave signal moves across space signal energy is stored in the reactive impedance of the transmission media and dissipated in the real resistance in a manner that is continuous and without disturbances for the electrical size of the signal during its travel in the transmission line.. When there is a discontinuity (of varying electrical properties) of the signal path is when the energy wave changes direction (instead of being transferred forward or dissipated) and energy is reflected back to the source by the transmission media. If the discontinuity is electrically small in both electrical size and electrical properties irt the traveling signal then little signal energy is reflected back to the source.

    A TDR shows the reflection signal changes with changes in the transmission line.
     
  3. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    115
    4
    Hello.

    Thanks for giving some comment.
    So..I think you said that the signal reflection happens when there is a discontinuity or erupt change of some electrical properties of the signal path. Those properties are wrapped to a quantity called the impedance. So when there is a sudden change of the impedance, we can expect the signal reflection in some degree there. In this sense, I think there should be some reflection at the interface from ZS to Z0 as these impedances are not matched.

    Could you tell me what does "electrical size" and "irt" you mentioned mean?
     
  4. nsaspook

    Expert

    Aug 27, 2009
    6,189
    6,974
    "Electrical size" is a shorthand term for electrical interactions with physical components. An analogy would be like a baseball sized stone. If you were riding a bicycle (a high frequency signal) it would be a serious discontinuity to the road surface because of the ratio of the mass/energy/size of the bike tire to the rock. If you were instead riding a huge strip-mine dump truck (a low frequency signal like 60hz) the discontinuity caused by the rock on the road would be negligible In Reference To the mass/energy/size of the dump truck tire.

    A proper term for transmission lines would be Electrical length. So if the electrical length of the interface (a short piece of wire and resistor) from ZS to Z0 is small then the phase difference across the interface is also small and the actual discontinuity seen by the signal energy is small because very little energy is stored to be reflected at this point.
     
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