Knee Frequency

The Knee Frequency for any digital signal is related to the rise(fall) time of its digital edges, but not to its clock rate;

Fknee=0.5/Tr

where Fknee = frequency below which most energy in digital pulses concentrates.
Tr = pulse rise time.

Shorter rise times push Fknee higher. Longer rise times pull Fknee lower.

1. Any circuit which has a flat frequency response up to and including Fknee will pass a digital signal practically undistored.
2. The behavior above Fknee of a digital circuit will have little effect on how it processes digital signals.


reference: High-Speed Digital Design