Although in special
circumstances the definition of pulse parameters might be changed to meet a certain need, the figure shown below
represents the standard used by both the analog and digital engineering communities. A positive pulse is presented
here, but everything applies equally for a negative pulse.
An ideal positive pulse is made up of a leading
edge with a 0-second rise time, a fixed amplitude voltage, current or power level of some duration, and then a
trailing edge with a 0-second fall time.
A real-world positive pulse is usually far from ideal - sometimes intentionally. The illustration below points
out some of the non-ideal aspects of pulses. Notice that the leading and trailing edges have rise and fall times,
respectively, that are not equal to zero time. Zero is nice (but unattainable in the real world) when timing of
pulses in a system of pulses is essential. Non-zero rise/fall times creates ambiguity in timing that can cause bit
errors to occur. An instance of when you might want some slope in the edges is in the case of audio circuits where
the high frequency content of a sharp edge can cause "chirp."
Another major non-ideal feature is overshoot
and ringing. Both phenomena are cause by a circuit that is not
critically damped. This situation is never useful, but can be
tolerated if the level sampling is don at the center of the pulse, after the signal has settled down.
Finally, amplitude droop from the beginning to the end of the pulse can cause the level detection to disqualify
the pulse if is is below the acceptable threshold when it is sampled. As with the other two characteristics, good
circuit design can either eliminate, reduce, or simply tolerate any or all of the non-ideal features.
Positive Pulse Waveform