Oscilloscope Bandwidth and Rise Time Calculator

The formulas

scope rise time tᵣ ≈ 0.35 / BW · amplitude error = 1 − 1 / √(1 + (f / BW)²) · measured tᵣ = √(signal tᵣ² + scope tᵣ²)

A scope behaves like a low-pass filter whose half-power point is its rated bandwidth. A sine at the bandwidth frequency shows up about 29 percent low. The calculator uses the common single-pole model, which is a good guide for the 20 to 200 MHz class of scopes.

The rules of thumb

• 3× rule: bandwidth at least three times your signal frequency keeps the amplitude error near 5 percent.
• 5× rule: bandwidth at least five times keeps it near 2 percent. Use this when you care about the measured amplitude.
• Fast edges: the scope rise time should be a third to a fifth of the signal rise time, otherwise the edge looks slower than it is. A square wave needs the harmonics, so judge it by its edge, not just its repetition rate.

Probe bandwidth counts too. The system bandwidth of scope plus probe is lower than either alone, so a 100 MHz scope on a 60 MHz probe is a 60 MHz setup. For how bandwidth, sample rate and memory depth fit together read Oscilloscope Bandwidth Explained and how to choose an oscilloscope.