the Bode plot
/ BOH-dee /
A circuit treats different frequencies differently, passing some and blocking or delaying others. A Bode plot is the picture of that behaviour: a graph of how much a circuit boosts or cuts a signal, and how much it shifts its phase, across a whole range of frequencies. It is a circuit's frequency fingerprint.
A Bode plot is actually two graphs stacked, both with frequency on a logarithmic horizontal axis (so each step is times-ten). The top one plots gain in decibels; the bottom plots phase shift in degrees. The log axes turn curved responses into nearly straight-line segments you can sketch by hand: below a filter's cutoff the gain is flat, and beyond it the gain falls along a straight slope. A simple first-order low-pass rolls off at 20 dB per decade, a tenfold drop in amplitude for every tenfold rise in frequency.
Bode plots are how engineers read a filter's cutoff and roll-off, judge an amplifier's bandwidth, and check feedback stability; too little phase margin where the gain crosses 0 dB warns of oscillation. Caveat: the tidy straight-line asymptotes are an approximation. The real curve rounds the corner, sitting 3 dB below the ideal right at the cutoff frequency, so use the asymptotes to sketch and the exact math when it counts.
The Bode plot of a simple RC low-pass is flat (0 dB) up to its cutoff, then bends down at 20 dB per decade; the phase slides from 0 toward -90 degrees, passing -45 degrees right at the cutoff.
Reading a first-order low-pass off its Bode plot.
The straight-line asymptotes are an approximation; the true response is 3 dB below the corner right at the cutoff frequency and rounds the bend.