The following tutorial explains the rise and fall times of digital signals. Various related terminlogies are discussed. The tutorial then explains the significance of the rise time and fall time along with the right way to measure these parameters.

Definition of Rise Time

Rise time is the difference between the time when the signal crosses a low threshold to the time when the signal crosses the high threshold. It can be absolute or percent.

Absolute Rise Time

In absolute rise time, the low and high thresholds are fixed voltage levels around the mid voltage level.

Percent Rise Time

In percent rise time, the low and high thresholds are percent levels, and are usually either 10% and 90% respectively or 20% and 80% respectively. The percent levels are converted to absolute voltage levels at the time of measurement by calculating percentages from the difference between the starting voltage level and the final settled voltage level.

Definition of Fall Time

Fall time is the difference between the time when the signal crosses a high threshold to the time when the signal crosses the low threshold. It can be absolute or percent.

Absolute Fall Time

In absolute fall time, the low and high thresholds are fixed voltage levels around the mid voltage level.

Percent Fall Time

In percent fall time, the low and high thresholds are percent levels, and are usually either 10% and 90% respectively or 20% and 80% respectively. The percent levels are converted to absolute voltage levels at the time of measurement by calculating percentages from the difference between the starting voltage level and the final settled voltage level.

Significance of Rise Time & Fall Time

This is best explained by comparing a square wave with a triangular wave. In an ideal square wave with 50% duty cycle, the rise time will be 0 and the signal will be above threshold for 100% of the half period time. In a symmetric triangular wave, this is reduced to just 50%. More severely affected is the total area above the threshold, which is reduced to 25% of that of square wave's. Though the information about loss of time above threshold is conveyed by many other parameters, the information about loss of area above threshold is only conveyed by rise and fall times.

Rise Time & Fall Time Requirements

The rise time & fall time should be small compared to the clock period. A factor of 10 is considered good. Very large rise or fall times have the risk of the cycles going undetected. Also, large rise or fall times mean that the signal will be hovering around mid level for too long, making the system highly susceptible to noise and multiple triggering if there is not enough hysteresis.

This might make you think that the faster rise & fall times are, the better the system is. Not really. Very fast rise or fall times are not free from trouble. They might cause severe ringing at the receiver resulting in reduction in voltage & timing margins or even double triggering. Or the fast edges can & will get coupled to the adjacent signal lines causing false triggering on them or reducing the voltage margins.

Measurement of Rise Time & Fall Time

The measurement system should have enough analog bandwidth to measure the edge times faithfully. This means that the oscilloscope and the probe together should have at least twice the bandwidth of the fastest edge rates to be measured. To estimate the highest frequencies of significant level in an edge, divide 0.35 with an estimate of the edge time (i.e. rise time or fall time).

If a digital oscilloscope is being used, an edge should get at least 5 samples for the measurements to be reasonably accurate. Normally, the lesser bandwidth the measuring system has, the larger the measured rise or fall time will be from the actual value. This means that the measured value for rise & fall rates will always be larger than the actual value. However, this is not always true. If the probe used for measurement is highly under-damped, it will result in ringing causing the measured signal to have faster rise & fall times than the actual signal.

You have to use cursors to measure the rise & fall times in analog oscilloscopes. In modern digital oscilloscopes, the measurements are automatically done by the oscilloscopes. However, all digital oscilloscopes measure only one edge in the whole acquisition (normally first). To have a higher confidence in the measurement, multiple measurements must be made. One way is to let the oscilloscope run in continuous run mode and let the statistics accumulate over time. This approach may be time consuming and will not measure consecutive edges.

The better way is to use a post processing software. These software are very fast and can acquire data from the oscilloscope automatically, process millions of data edges in just a few seconds, display the results graphically in time domain as well as in frequency domain, with statistics along with the time stamp of max and min values. You can even save waveforms from the oscilloscope for post processing by the software.

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