Rise Time → Bandwidth & Knee Frequency
Estimate signal bandwidth (0.35/tr) and knee frequency (0.5/tr) from the rise time.
Worked example
Input: Rise time (10–90 %) 1 ns
Result: Signal bandwidth ≈ 0.35/tr 350 MHz, Knee frequency ≈ 0.5/tr 500 MHz
Result: Signal bandwidth ≈ 0.35/tr 350 MHz, Knee frequency ≈ 0.5/tr 500 MHz
Formula
BW ≈ 0.35 / t_r
f_knee ≈ 0.5 / t_r
How it works
What a digital signal does to EMI is governed more by its rise time than by its clock frequency. Spectral energy stays high up to about the knee frequency and then rolls off. It gives you a starting point for how far up in frequency emission countermeasures must reach.
Practical tipSwapping in a faster driver at the same clock shortens the rise time and raises high-frequency emissions. Slowing unnecessarily fast edges with a series resistor of a few tens of ohms is one of the cheapest EMI fixes.
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Results and summaries are for reference. For certification and test reports use the latest official standard text and calibrated instrument data. Last updated: 2026-10-10