High speed Designs - Part 19 (What is Knee Frequency?)

For any digital signal, power is concentrated from DC to a certain frequency which is called Knee Frequency.

Generally, we tend to understand that, the signal power at higher frequencies, depends on the signal frequency being transmitted. This is not the case and it depends on the Rise time/Fall time. 

Knee Frequency = 0.5/Rise Time

So, for a 1 ns rise time signal, the knee frequency is 500 MHz. So, for 1 ns rise time signal, there won't be significant power in the frequencies > 500 MHz.