Engineers occasionally forget that a delay line acts as a multipole, low pass filter. This becomes a problem when designers attempt to delay signals with higher frequency characteristics than the delay line is capable of passing. Especially at higher frequencies, delay accuracy can change significantly with a change in the input waveform due to variance in the group delay. Only then can the vendor supply the correct product. Failure to specify the falling edge accuracy is probably the most common omission when specifying a delay line.
Many designers wish to maintain a specific pulse width from input to output.
In order to maintain the duration of the input pulse width, it is imperative to specify both the rising and falling edge timing accuracy. Don't mix terminology. Electronic engineers already know that the overall delay time is the time duration between the reference or threshold level of the leading edge of the input pulse, to the reference or threshold level of the leading edge of the output pulse. Analog delay lines are measured at the 50 percent point of the signal, while digital delay lines use a fixed voltage level, typically the logic threshold voltage. Additional terms that need to be carefully defined include leading and trailing edges, versus rising and falling edges.