Low impedance delay lines have the characteristic of relatively high capacitive elements in relation to low inductive elements. High impedance lines have relatively low capacitive elements and high inductive elements. Consider a short time delay, high impedance delay line with ten taps whose external circuitry adds 2pf per tap, for a total of 20pf of capacitance. This added capacitance will affect delay time accuracy and impedance on the delay line, in some cases, changing delay accuracy or impedance by 20 percent. Specifying delay lines for high frequencies requires special considerations.
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.