Click to place poles (×) and zeros (○) on the z-plane
Infinite Impulse Response: IIR filters have feedback, so their impulse response theoretically lasts forever (though it decays for stable filters).
Poles (×): Locations where the transfer function goes to infinity. Create peaks in the frequency response. Must be inside the unit circle for stability.
Zeros (○): Locations where the transfer function goes to zero. Create notches in the frequency response. Can be anywhere.
Stability Rule: A digital IIR filter is stable if and only if all poles lie strictly inside the unit circle |z| < 1.