# IIR Filter Designer > Design digital IIR filters (Butterworth/Bessel, lowpass/highpass): pick order and cutoff, read the frequency response, poles/zeros and biquad coefficients — with CMSIS-DSP-ready export. Practice: elettronica Status: Proto v0.1 Tag: IIR, Biquad, SOS, DSP, CMSIS-DSP Method, assumptions and limits: Page: --- Designing digital IIR filters as a cascade of second-order sections (SOS). Set type (lowpass/highpass), family (Butterworth or Bessel), order (2–8) and cutoff relative to the sample rate: get the magnitude and phase response, the pole-zero map on the unit circle, the biquad coefficient table and the coefficient export for arm_biquad_cascade_df2T (CMSIS-DSP). Coefficients validated against scipy.signal. This file is a **descriptor**, not the tool's documentation: it says what the tool computes and by what method, in summary. Formulas, assumptions, validity conditions and limits are in the wiki linked above. The tool runs in the browser at the page given.