Also known as: IIR, infinite impulse response filter

An IIR (infinite impulse response) filter computes each output from both recent inputs and past outputs — it has feedback.¹ That recursion achieves a sharp frequency response with far fewer coefficients than a FIR filter, at the cost of nonlinear phase and a need to watch stability.

An IIR filter feeds past outputs back into the input, giving a sharp response from very few terms.

Overview

IIR designs (often built from cascaded biquad sections) suit narrowband tasks like DC blocking and the loop filters in AGC and timing recovery, where efficiency matters more than phase linearity.

Sources

1. Infinite impulse response — Wikipedia, on the recursive, feedback-based filter and its stability trade-offs. ↩

See also

• FIR filter
• Digital filter
• Automatic gain control (AGC)

Learn it step by step

• Filtering & decimation

From the blog

• SDR Internals, Part 4: DSP foundations — filters, NCO & AGC