Pulse Duration or Risetime?

A detector’s temporal performance is often specified by either pulse duration or risetime. Which one of these parameters is appropriate for your application?

Pulse duration is best used when you are actually measuring pulses, i.e. signals that turn on and then return to zero. The pulse duration of a detector tells you the shortest pulse you could ever expect to see output from the detector. For good resolution, you need to select a detector whose FDHM is at least three times shorter than the pulse you expect to measure.

Risetime is the parameter of choice when you are measuring either rising or falling edges. This type of measurement is especially common in digital communications systems where bit streams are comprised of an endless series of rising and falling edges. Once again, for good resolution in diagnostic applications, the risetime of a detector should be at least three times shorter than the risetime you expect to measure.

Clearly, pulse duration and risetime are related quantities (see figure). Mathematically, the risetime of a detector can be obtained by integrating its pulse response. Clean pulses without tails or ringing, approximate a Gaussian shape. Such pulses have risetimes (10-90%) that are only 10% longer than the FDHM. In this case, the difference between the two values is negligible.

However, when pulse shapes deviate from the ideal, the difference between pulse duration and risetime can indeed become significant. Pulses with positive tails produce longer risetimes with less bandwidth, while pulses with negative ringing produce shorter risetimes and have enhanced bandwidth.