Notch Filter (Band-Reject) Calculator

The twin-T (twin-tau) notch network was developed in the 1930s as a passive band-reject filter for telephone-line filtering. Its symmetric T-of-R / T-of-C structure produces a precise null at f₀ = 1/(2πRC) — relied on only for component-value matching and not the absolute values.

Biomedical-instrumentation engineers adopted twin-T heavily in the 1960s-70s for 50/60 Hz mains rejection in EEG, ECG, and EMG amplifiers where body skin potentials pick up hum from nearby mains wiring. A buffered twin-T with Q ≈ 20 gives 40-60 dB null with only 3-dB notch-skirt at ±5 Hz from the target frequency.

Modern digital filtering in DSP-based instruments has largely replaced analog twin-T notches, but the passive version remains popular for hobbyist audio (anti-feedback on guitars), electric-fence controllers (suppress radiated power-line harmonics), and any budget-critical instrumentation where an op-amp would be overkill.

Enter notch frequency f₀ and capacitor value C. The tool computes R = 1/(2π·f₀·C), then derives R3 = R/2 and C3 = 2C for the shunt pair of the twin-T. For higher Q (target Q > 1), add an op-amp buffer and positive feedback via the feedback factor k = (2Q-1)/(2Q) applied at the shunt junction.

For deep notches, use ±1% metal-film resistors (R and R/2 matched), and COG/NP0 ceramic caps (C and 2C matched). Mismatched components spread the null over a wider bandwidth or shift the center frequency. Everything runs client-side.