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Reflected voltage pulses

  1. Apr 1, 2013 #1
    Why are voltage pulses reflected back over long transmission lines or in circuits having high frequency? Does this follow basic principles of electronics( Kirchoff's law, etc. ) ?
  2. jcsd
  3. Apr 1, 2013 #2
    The short answer is because the signal encounters an impedance discontinuity. While you wait for the longer answer you can have a look at this mesmerizing video that someone posted a link to a while back:

  4. Apr 1, 2013 #3
    Could someone explain how this wave nature doesn't go against the basic laws of electronics ?
  5. Apr 1, 2013 #4

    jim hardy

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    That's why they teach us DC first, it's easier to visualize. Charge acts not unlike like an incompressible fluid moving through tubes. They hardly mention fields.

    Then we learn low frequency AC. That introduces complex arithmetic - sine functions and all that.
    That's where they start with fields, to explain inductance and capacitance.

    So long as the frequency is low we don't have to consider wave phenomena.
    The higher the frequency the more electric energy behaves like light waves. It'll reflect, make interference patterns, and travel through air.

    Try a search on transmission line theory.
    ARRL Antenna Handbook is a good, practical reference.

    old jim
  6. Apr 1, 2013 #5

    Why don't we see reflection in short channels or while applying DC voltage? Normally in open circuits the applied voltage is the voltage observed across the entire length of wire but by reflection property we should actually observe twice the voltage than what is applied [ incident + reflected]
  7. Apr 2, 2013 #6
    You do see reflections in short channels if you are operating at a high enough frequency. ALL electrical lines have some reflection but it doesn't always matter. In practice you start seeing reflections when the line length is comparable to about half the wavelength of the incident wave or so. In a logic circuit, you can start having problems if the time it takes for the reflected wave to show up is close to the propagation delay of the driving gate. In this case you may need to go to balanced logic circuits, which are used in practice in very high speed digital circuits.
  8. Apr 2, 2013 #7


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    Actually, you soon become aware of the time taken for signals to pass through an electronic circuit, once you try to build an amplifier. It is extremely easy to build an amplifier which turns out to be an oscillator because the feedback loop you included (trying to make it more linear, for instance) introduces an embarrassing amount of delay (phase shift). Even at relatively low frequencies, the wave nature of signals can give you trouble.
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