How to read a Bode plot
A Bode plot is two graphs that show how a linear system responds to sinusoids across frequency. Reading it well lets you judge bandwidth, resonance, and stability at a glance. This guide walks through each part.
The two graphs
The top graph is magnitude in decibels, computed as 20·log10|H(jω)|. The bottom graph is phase in degrees, the angle of H(jω). Both share a horizontal frequency axis drawn on a logarithmic scale, so each evenly spaced division is a factor of ten (a decade).
Corner frequencies and roll-off
Each pole bends the magnitude curve downward by an extra −20 dB per decade and contributes −90° of phase; each zero bends it upward by +20 dB per decade and adds +90°. The frequency where a factor starts to act is its corner (or break) frequency. A first-order low-pass, for example, is flat until its corner, then rolls off at −20 dB/decade while the phase passes through −45° at the corner.
Reading stability margins
The gain crossover frequency is where the magnitude crosses 0 dB; the phase margin is how far the phase there is above −180°. The phase crossover frequency is where the phase crosses −180°; the gain margin is how far below 0 dB the magnitude is there. Positive margins mean the closed loop is stable, with larger margins giving more robustness.