Twin‑T RC Notch Filter Calculator
Use this calculator to find notch frequency, resistor value, or capacitor value for a standard Twin‑T notch filter. Formula used: f0 = 1 / (2πRC).
What is a notch filter?
A notch filter (also called a band-stop filter) removes a very specific frequency while allowing most other frequencies to pass through. It is commonly used to suppress narrowband interference such as 50/60 Hz mains hum, mechanical resonance tones, or persistent noise from sensors and measurement systems.
One of the most practical analog designs is the Twin‑T RC notch filter, which uses only resistors and capacitors. It is easy to build and excellent for applications where you want a targeted rejection frequency without using inductors.
How this calculator works
This tool uses the ideal Twin‑T notch relationship:
You can solve for any one variable:
- Frequency: f0 = 1 / (2πRC)
- Resistor: R = 1 / (2πf0C)
- Capacitor: C = 1 / (2πf0R)
It also returns a recommended ideal component set for the full Twin‑T network:
- R1 = R2 = R
- R3 = R/2
- C1 = C2 = C
- C3 = 2C
Example: 60 Hz hum rejection
Design goal
You want to attenuate 60 Hz line noise in a sensor front-end using C = 100 nF capacitors.
Calculation
Using the resistor equation, R = 1 / (2πf0C):
- f0 = 60 Hz
- C = 100 nF
- R ≈ 26.5 kΩ
Your ideal Twin‑T values become approximately:
- R1 = R2 = 26.5 kΩ
- R3 = 13.25 kΩ
- C1 = C2 = 100 nF
- C3 = 200 nF
Practical design notes
1) Component tolerance matters
Notch depth is sensitive to mismatch. For deeper rejection, use 1% resistors and tight-tolerance film capacitors when possible.
2) Passive vs active notch filters
A passive Twin‑T can attenuate the notch frequency well, but insertion loss may reduce overall signal level. Active versions (with an op-amp) can improve notch sharpness and restore gain.
3) Real-world tuning
When exact nulling is critical, include a trim resistor (or parallel trim network) to fine-adjust the center frequency during testing.
4) Keep layout clean
In low-frequency precision designs, long traces and noisy grounds can reduce filter performance. Place components close together and use a stable ground reference.
When to use a notch filter
- Removing mains hum from audio or instrumentation circuits
- Suppressing known mechanical vibration frequencies
- Cleaning up ADC input channels in embedded systems
- Reducing narrowband interference in biomedical signals
FAQ
Is a notch filter the same as a low-pass filter?
No. A low-pass filter removes high frequencies broadly. A notch filter removes only a narrow range around a specific center frequency.
Can I use this for 50 Hz power-line noise?
Yes. Set your target frequency to 50 Hz and choose either R or C based on convenient component values.
How do I get a deeper notch?
Use accurate components, keep the recommended ratio values, and consider an active notch topology if very high attenuation is required.
Final thoughts
This notch filter calculator is designed for fast early-stage design and educational use. It gives a strong starting point for Twin‑T RC filters, after which you can fine-tune values based on component availability, tolerance analysis, and measured performance in your actual circuit.