I have been looking for cause of signal reflections in transmission lines. Everywhere it is concluded that the reason is impedance mismatch. I can understand if the impedance changes in the path of the signal travels, then reflection will occur, but what I could not understand is the physics behind it.
How does including a series resistor of the characteristic impedance suppress the reflections, since the direction of reflected voltage is opposite? Does it mean the point at which the impedance changes becomes high in potential or anything else?
Can anyone give directions to understand the physics behind this?
Best Answer
With electrical transmission lines, it all has to do with the speed of light being finite, thus so is the speed of EM propagation in a wire. You can think of a wire as a long series of infinitesimal capacitors (connected by infinitesimal inductors). If you start charging the capacitors at one end, you have to keep pumping charge into the wire to charge more and more of the infinitesimal little distributed capacitors down the wire (at the speed of EM propagation).
Then a problem occurs when you get to the end of the wire.
If shorted, the last capacitor can't charge, and it in turn then discharges the capacitor one back. This discharging of capacitors then travels back to the source (at the speed of EM propagation) as a negative voltage reverse wave that cancels out the forward positive wave.
If open, remember there is still charge being pumped into the line. It has to go somewhere, so the "momentum" of the current (due to distributed inductance of the wire) charges up the last capacitor to double the voltage of the rest of the line capacitance. But this last capacitor is connected to all the ones before it. So this wave of charging to double the voltage then propagates backwards on the wire as a positive reflection wave.
Somewhere in between a short and an open at the end is a nice medium value of impedance which can absorb the wave of current hitting the end without either under or overcharging that last bit of capacitance at the end. Thus leaving no change in voltage to propagate back along the transmission line. That terminating impedance just happens to be the characteristic impedance of the transmission line, which is determined by the distributed capacitance and inductance of the wire (and its surroundings: permeability, ground plane or return geometry, coax shield, et.al.)
Pretty much the same thing occurs with air pressure waves in a tube or pipe (organ), or when whipping one end of a rope sideways. Different rope waveforms occur depending on whether the rope is tied or loose at the opposite end. etc.
(Added: For transmission lines where there is a discontinuity or mismatch in impedance somewhere in the middle, you can think of it as a superposition of two transmission lines, one line with no discontinuity over the total length, plus one shorter line with an open or short at the discontinuity. The reflection will relate to the ratio of the two superpositions.)