Lesson 1: Section 7: Wave Bounce Diagrams for Digital signals


Section 7: Wave Bounce Diagrams for Digital Signals

Now imagine if the switch in the previous figures, rather than staying closed, opens and closes at a regular interval. Instead of a steady state voltage, voltage waves would continue to bounce around back and forth along the transmission line. The reflections caused by this square wave traveling along a transmission line is a real problem with high speed digital lines. When everything is matched (Figure 1.4.1) there are no voltage bounces. When impedances are mismatched (Figure 1.5.1), voltage waves bounce back and forth. Some of the energy is transmitted to the load, and some of the energy is reflected back to the source.

If the transmission line is short enough, this problem is mitigated: the voltage settles more quickly if it doesn't have a long distance to bounce around.

Digital Signal Voltage Wave Bounce with Mismatch

The energy that is transmitted to the load is (generally) useful energy. The energy that is reflected back is (generally) wasted energy. How much of the energy is wasted? According to the Wave Bounce Diagram 0.1V was reflected off of the first bounce, but some portion of that was reflected back to the source, so the actual amount of energy wasted is proportional to the voltage actually received by the source, 0.0828V from the first bounce plus 0.0024V from the third bounce, seen in Figure 1.7.1.

Figure 1.7.1: Wave Bounce Diagram showing wasted energy

Main Point

Because the waves are constantly varying, tracking how voltages are transmitted and reflected are much more difficult for digital analysis compared to what goes on in the steady state. Because the voltages keep changing, if there is a mismatch some amount of energy is periodically being reflected back to the source. Not only is this wasted power, the reflected voltages interfere with the digital output- potentially corrupting the signal. This is exactly why USB cables cannot be more than 5 meters long.