gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
110405-0606 EDT
ActionDave:
A fast rise pulse generator driving a cable of known length. The cable is unterminated at the far end. Actually the far end is very close physically to the pulse generator, a foot or so. An oscilloscope synchronized from the pulse generator. The left side of the screen is 0 time delay. The vertical input is connected to the open circuit (unterminated) far end and the time for the pulse to travel to the far end is measured on the horizontal axis of the scope. For the measurement made in this way it is unimportant that the far end is unterminated. The velocity of propagation determines the travel time to the far end.
Leaving the far end unterminated will cause energy to be reflected and so the whole measurement could actually be made from the input end because the reflected energy causes a step in the waveform seen at the input. This is the basis of TDR.
The material of the transmission line, physical structure, and surrounding material determine the propagation velocity of the energy vs the speed of light.
Light energy travels at approximately 186,000 miles per second, 982 ft per microsecond, in free space in a pure vacuum. When light encounters a material like glass or water it slows down, and this is the basis of the theory of lenses. When light strikes a flat water surface at an incident angle that is not perpendicular to the surface its angle and speed of propagation in the water changes from that outside the water.
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ActionDave:
A fast rise pulse generator driving a cable of known length. The cable is unterminated at the far end. Actually the far end is very close physically to the pulse generator, a foot or so. An oscilloscope synchronized from the pulse generator. The left side of the screen is 0 time delay. The vertical input is connected to the open circuit (unterminated) far end and the time for the pulse to travel to the far end is measured on the horizontal axis of the scope. For the measurement made in this way it is unimportant that the far end is unterminated. The velocity of propagation determines the travel time to the far end.
Leaving the far end unterminated will cause energy to be reflected and so the whole measurement could actually be made from the input end because the reflected energy causes a step in the waveform seen at the input. This is the basis of TDR.
The material of the transmission line, physical structure, and surrounding material determine the propagation velocity of the energy vs the speed of light.
Light energy travels at approximately 186,000 miles per second, 982 ft per microsecond, in free space in a pure vacuum. When light encounters a material like glass or water it slows down, and this is the basis of the theory of lenses. When light strikes a flat water surface at an incident angle that is not perpendicular to the surface its angle and speed of propagation in the water changes from that outside the water.
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