Signal Knee Frequency

Formula

Description

The knee frequency is the frequency below which most of a digital edge’s energy is concentrated; above it the spectrum rolls off rapidly. It is the practical bandwidth a signal path must support to preserve edge fidelity. Unlike the −3 dB bandwidth (0.35/tr), the knee marks where the edge’s significant harmonic content ends, and it sets the frequency at which transmission-line effects must be considered.

Variables

• fknee — Knee frequency (Hz)
• tr — Signal rise (or fall) time, 10–90% (s)

Practical Notes

Route and terminate any interconnect that matters up to fknee. A 1 ns edge has a knee at 500 MHz—so even "slow" logic with fast edges needs controlled impedance.