Really Big Transmission Line

Blue is the input signal (a square wave) Orange is the voltage after the source impedance, connected right at the start of the transmission line (top left) Green is the voltage level around halfway down the transmission line Red is the voltage at the termination, connected all the way at the end of the transmission line (bottom left) This is a transmission line with a 50 Ohm intrinsic impedance (found by taking sqrt(L/C) in Ohms) with a matching 50 Ohm source impedance and 50 Ohm termination at the end of the line. In this setup, there are no serious reflections and the voltage on the line simply rises to half the input signal; exactly what’s expected of a 1:1 ratio voltage divider. The switches at the end of the line can be configured for the extremes of either an open circuit (infinite impedance) or a short circuit (0 Ohm impedance) to show the effect of reflections. An open termination results in a positive reflection that returns to the source and increases the voltage up to the supply level. A shorted termination sends a negative reflection which will lower the voltage at the line’s input to the negative of the forward going signal (0.5V as the source sees its 50 Ohm impedance in series with the 50 Ohm transmission line impedance, making the 1/2 voltage divider)

published 7 months ago