20 Chapter 1 Pulses The pulse is a waveform, but one that you might not recognize as a wave because the time of the pulse is very short compared to the time between pulses. Figure 1.11 illustrates three different pulse shapes, all of which share a very sharply rising portion, the leading edge. For a negative pulse, this leading edge would be a sharp fall in voltage. Definition A pulse is a rapid change in voltage which is of very short duration compared with the time between pulses. For example, a pulse might repeat at a rate of 1 kHz, 1000 pulses per second. The actual pulse might have a duration, a pulse time, of only 10 ms, so that the change in voltage lasts only for 10 ms in the 1 ms (millisecond) between pulses. That makes the time of the pulse (its duty cycle) 1/100 of the time between pulses, and these are fairly typical figures. Pulses are used for timing, and they have the advantage that they use very little energy because the change in voltage is so short. A pulse can be used to start an action, to stop an action, or to maintain an action (such as keeping a wave in step, synchronized, with the pulses). Modern digital electronics systems, particularly computers, rely heavily on the use of pulses, and when we work on these systems we are not greatly concerned about waveshapes, only about pulse timing. There are many things that can change a waveshape, making it very difficult to preserve the shape of a wave. By contrast, it is more difficult to upset pulse timing by any natural means, so that circuits which depend on pulse timing are more reliable in this respect than circuits that depend on waveshapes. Figure 1.11: Examples of pulse waveforms. The common factor is a sharp leading edge