Plan twin-T networks with clear component ratios. Check frequency drift from realistic part tolerances today. Save results as tables for faster design reviews everywhere.
A balanced Twin-T notch is built by paralleling a low-pass T and a high-pass T network. When the ratios are set for balance, the notch (null) frequency is:
Using the base values R and C, ideal component ratios are: R1=R2=R, C3=2C, C1=C2=C, and R3=R/2.
| Base R | Base C | Notch f0 | R3 (R/2) | C3 (2C) |
|---|---|---|---|---|
| 10 kΩ | 10 nF | ≈ 1591.55 Hz | 5 kΩ | 20 nF |
| 6.8 kΩ | 10 nF | ≈ 2340.52 Hz | 3.4 kΩ | 20 nF |
| 33 kΩ | 4.7 nF | ≈ 1026.95 Hz | 16.5 kΩ | 9.4 nF |
A Twin-T notch filter is a passive network that strongly rejects a narrow band of frequencies while passing frequencies below and above the notch. It is widely used to remove tonal interference such as mains hum, motor whine, or a single vibration line in sensor signals. The network is formed by paralleling a low-pass T section and a high-pass T section so their impedances balance and cancel at the notch frequency.
In the balanced case, the notch frequency follows f0 = 1/(2πRC). The calculator lets you solve for f0, R, or C, then generates the ideal ratios R1=R2=R, C1=C2=C, R3=R/2, and C3=2C. This ratio matters as much as the absolute value.
For audio and instrumentation, common base resistances fall between 1 kΩ and 200 kΩ. Very small resistors increase loading and noise current, while very large resistors increase sensitivity to leakage and stray capacitance. Base capacitances in the 1 nF to 470 nF range often keep parasitics manageable.
The notch depth depends heavily on matching between parts, not only their nominal values. A 1% mismatch can noticeably reduce attenuation at f0. The calculator estimates a worst-case frequency window using the entered R and C tolerances. For deeper notches, use 0.1% resistors, film capacitors, or measure and pair parts.
A passive Twin-T assumes the source and load do not significantly disturb the network. In practice, a low input impedance load will broaden the notch and shift the center frequency. Buffering with a high input impedance stage (or using an active notch topology) preserves the calculated response.
Many designs place the Twin-T network in the feedback path of an amplifier. This can increase attenuation and sharpen the notch without requiring extreme component values. Even in active forms, the same base frequency relationship is used, so the computed component set remains a solid starting point.
Typical targets include 50 Hz or 60 Hz hum, 100/120 Hz rectifier ripple, and fixed tones such as 1 kHz calibration artifacts. For example, choosing R = 10 kΩ and C = 10 nF gives f0 ≈ 1591.55 Hz. The example table below the calculator shows several practical pairs.
After selecting parts, verify the notch using a sine sweep or a stepped tone test around f0. If the notch is shallow, first check ratio accuracy (R/2 and 2C) and matching between the two series components in each T section. Export results to CSV for build notes and repeatable documentation.
1) What is the notch frequency in a balanced Twin-T?
The ideal notch frequency is f0 = 1/(2πRC), where R and C are the base values used to build the two T sections with the standard ratios.
2) Why does the calculator set one resistor to R/2 and one capacitor to 2C?
Those ratios balance the impedances of the low-pass and high-pass T networks at the notch point, creating deep cancellation at f0.
3) How do tolerances affect the result?
Tolerances shift f0 and reduce notch depth because mismatches prevent perfect cancellation. Better matching generally improves attenuation more than changing nominal R or C.
4) Can I use this network directly between two stages?
You can, but the source and load impedances should be high compared to the network. If loading is significant, buffer the filter or consider an active notch approach.
5) Which component type is recommended for capacitors?
Film capacitors are commonly preferred for stability and low loss. Ceramic parts can work, but temperature and voltage coefficients may shift the notch more in precision applications.
6) How can I deepen the notch without changing the frequency?
Improve matching: use tighter tolerance parts, measure and pair components, and keep lead lengths short to reduce parasitics. Active feedback implementations can also increase attenuation.
7) What frequency range is practical?
It works from sub-audio to hundreds of kilohertz, but parasitics and loading matter more at higher frequencies. Choose moderate resistor values and realistic capacitors to keep stray effects small.