fall time (tf)
The fall time is used for pulses and square-wave signals. It is the time that elapses between the 90% and 10% value of a level change, referred to the highest and lowest level of the pulse, i.e. to the pulse top and the pulse bottom.
The overshoot and undershoot are not considered when determining the fall time. The fall time is directly related to the low-pass behavior of the transmission line or a quadripole through which the pulse passes.
As an approximation, the quotient 350/ bandwidth of the lowpass can be applied, with the bandwidth in megahertz( MHz) and the fall time in nanoseconds (ns). For example, if a lowpass filter has a cutoff frequency of 5 MHz, then a pulse with an extremely short decay time will have a decay time of about 70 ns after passing through the lowpass filter.