Switching devices might have quite different switching patterns in the very beginning or ending stages of their switching range, but usually all have quite similar patterns during their main transition.Īnd, the 10% and 90% points may more closely resemble the switching “threshold” levels of the associated logic devices. You might ask: Why do we refer to the 10% to 90% range of amplitude for rise time and not the 0% to 100%? Or, why do some use the 20% to 80% measure for rise time? The answer may be somewhat less than satisfying, but it goes like this: Almost all switching devices follow a switching pattern similar to that shown by the rising line in Figure 1, but they may differ significantly in the very first or last portions of their switching pattern.įor example, one type of device may reach 100% very quickly, but another device may roll off as it approaches the 100% level, and actually may not reach 100% for several moments (we sometimes say it may approach 100% asymptotically). Rise time of a waveform and the period of an underlying sine wave. We usually specify the rise time as the time between the 10% and 90% points in this transition (see Figure 1), but some spec sheets will specify it as the time between the 20% and 80% points.įigure 1. The rise time (or alternatively the fall time) of a signal is defined as the time it takes the waveform to transition from one peak level to the other. A sine wave of 100 Hz (many years ago we called it “cycles-per-second,” which actually had an intuitive meaning!) repeats itself (goes through its cycle) 100 times each second. The frequency of a waveform is the number of times it repeats (cycles) in one second.