Notch Filter (Band-Reject) Calculator
Design a twin-T or active notch filter to reject a narrow frequency band — typically 50/60 Hz mains hum or audio feedback. Solves R, C for target notch frequency.
How to Use
- Enter the notch frequency (the frequency you want to reject).
- Pick a convenient resistor or capacitor; tool computes the other.
- For deep notches (high Q), use active version with op-amp positive feedback.
Show Work
Formulas (Twin-T)
History
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.
About This Calculator
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.
About the Notch Filter (Band-Reject) Calculator
Whether you are at a desk or on your phone, the Notch Filter (Band-Reject) Calculator makes electronics and circuit design easy — and it is completely free. Design a twin-T or active notch filter to reject a narrow frequency band — typically 50/60 Hz mains hum or audio feedback. Solves R, C for target notch frequency.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
Twin-T topology?
Passive network: R1 = R2 = R, R3 = R/2 (shunt to ground), C1 = C2 = C, C3 = 2C. Deep notch at f₀ = 1/(2πRC). Passive Q ≈ 0.25.
Active version?
Buffer the passive twin-T with an op-amp, then add positive feedback from the output to the shunt junction. Adjusts Q from 0.25 to > 50.
Q vs depth?
Higher Q = narrower, deeper notch but more sensitive to component tolerance. Q = 10 is a good compromise for 60 Hz hum rejection.
How do I use the Notch Filter (Band-Reject) Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
60 Hz Mains Hum
Biomedical EEG/ECG amps: high-Q 60 Hz notch to reject mains pickup from electrodes.
50 Hz (Europe)
Audio recorder noise gate: notch at European mains frequency.
Audio Feedback
Live sound: narrow notch at the feedback frequency to kill howling without affecting tone.
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