Signal Knee Frequency
fknee = 0.5 / tr
Quick answer
This calculator solves fknee = 0.5 / tr for Knee Frequency. With Rise Time 10 ns, the result is 50 MHz. It solves in 2 directions, so you can enter the values you have and get the one you need.
Calculator
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.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| fknee | Knee Frequency | MHz | Hz, kHz, MHz, GHz |
| tr | Rise Time | ns | ps, ns, µs, ms, s, min, h |
Step by step
- 1. What you know: Rise Time 10 ns
- 2. The relation: fknee = 0.5 / tr
- 3. Substitute: tr = 10 ns
- 4. Result: Knee Frequency = 50 MHz
Worked examples
Minimum range
Given: Rise Time 100 ps
Result: Knee Frequency = 5 GHz
Low range
Given: Rise Time 1 ns
Result: Knee Frequency = 0.5 GHz
Typical
Given: Rise Time 10 ns
Result: Knee Frequency = 50 MHz
High range
Given: Rise Time 100 ns
Result: Knee Frequency = 5 MHz
Maximum range
Given: Rise Time 1 µs
Result: Knee Frequency = 0.5 MHz
Solving for each variable
Solve for Knee Frequency
Rise Time 100 ps → 5 GHz
Solve for Rise Time
Knee Frequency 100 kHz → 5 µs
How Knee Frequency changes with Rise Time
| Rise Time (input) | Knee Frequency (result) |
|---|---|
| 1 ns | 0.5 GHz |
| 2.5 ns | 0.2 GHz |
| 5 ns | 100 MHz |
| 7.5 ns | 66.67 MHz |
| 10 ns | 50 MHz |
| 15 ns | 33.33 MHz |
| 20 ns | 25 MHz |
| 40 ns | 12.5 MHz |
| 70 ns | 7.143 MHz |
| 100 ns | 5 MHz |
The same result in other units
| Unit | Value |
|---|---|
| kHz | 50000 |
| MHz | 50 |
| GHz | 0.05 |
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<p><a href="https://vbatpower.com/formulas/knee-frequency/">Signal Knee Frequency</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate Signal Knee Frequency?
Use fknee = 0.5 / tr. With Rise Time 10 ns, Knee Frequency works out to 50 MHz.
What is the formula for Signal Knee Frequency?
The formula is fknee = 0.5 / tr. The knee frequency is the frequency below which most of a digital edge’s energy is concentrated; above it the spectrum rolls off rapidly.
How do you find Rise Time when you know the other values?
Rearrange for Rise Time: with Knee Frequency 100 kHz, Rise Time is 5 µs.
What units does Signal Knee Frequency use?
Knee Frequency is shown in kHz, MHz, GHz. Values are held in SI base units internally, so you can mix prefixes without converting by hand.
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