Example Data Table
| Target | Base C | Base R | R3 | C3 |
|---|---|---|---|---|
| 50 Hz | 100 nF | 31.831 kΩ | 15.915 kΩ | 200 nF |
| 60 Hz | 100 nF | 26.526 kΩ | 13.263 kΩ | 200 nF |
| 1 kHz | 10 nF | 15.915 kΩ | 7.958 kΩ | 20 nF |
Formula Used
The ideal balanced twin T notch frequency is:
f0 = 1 / (2πRC)
The angular frequency is:
ω0 = 2πf0
The passive quality factor is estimated as:
Q ≈ 0.25
With optional feedback, this calculator estimates:
Q ≈ 0.25 / (1 - k)
The estimated bandwidth is:
BW = f0 / Q
The balanced component ratios are R1 = R2 = R, R3 = R / 2, C1 = C2 = C, and C3 = 2C.
How to Use This Calculator
Choose the calculation mode first. Select parts mode when you already know R and C. Select solve R when the target frequency and capacitance are fixed. Select solve C when the target frequency and resistance are fixed.
Enter tolerance and temperature data for a practical estimate. Add source and load resistance values to check loading risk. Press the calculate button. The result appears above the form. Use the CSV or PDF button to save the output.
Advanced Twin T Notch Filter Guide
What Is a Twin T Notch Filter?
A twin T notch filter removes one narrow frequency from a signal. It uses two T networks in parallel. One path is resistive. The other path is capacitive. At the notch frequency, their signals cancel. The output then becomes very small. This makes the circuit useful for hum removal, tone cleanup, and measurement work.
Why Component Ratios Matter
The classic balanced form uses R, R, and R over two. It also uses C, C, and two C. These ratios place both T networks at the same cancellation point. Small errors reduce the depth quickly. A one percent mismatch may leave visible residue. A five percent part set may create a shallow notch. Precision parts help more than random trimming. Trimmers can improve final rejection after assembly.
Design Choices
This calculator lets you solve from actual parts. It can also find resistance from a target frequency. It can find capacitance when resistance is fixed. That helps when only stocked parts are available. The tool estimates Q, bandwidth, angular frequency, and period. It also lists the six matched component values. Tolerance and temperature entries show practical drift risk. Source and load values warn about loading problems.
Advanced Use
A passive twin T has low Q. Its notch is broad. Active feedback can sharpen the response. The feedback factor in this tool gives an estimated improvement. Keep it below the stable limit. Very high feedback can cause ringing or oscillation. Use a buffer before and after the network when possible. A low source impedance and a high load impedance preserve the calculated response.
Good Applications
Use this design for 50 Hz or 60 Hz hum filters. It also fits tone rejection, oscillator cleanup, and sensor noise removal. Audio circuits need careful level testing. Instrument circuits need shielding and stable parts. Always check the result with a sweep or scope. The calculator gives a strong starting point.
Export And Documentation
The result table is useful during prototyping. Save the CSV file for spreadsheets. Save the PDF file for reports. Record chosen parts before building. Compare calculated and measured notch points. Then adjust matched pairs slowly. This process improves repeatability and makes later maintenance easier for every revision.
FAQs
What does a twin T notch filter do?
It strongly reduces one selected frequency. It is often used for hum rejection, tone removal, and narrow interference control.
What is the main notch frequency formula?
The balanced formula is f0 = 1 / (2πRC). R is the base resistance. C is the base capacitance.
Why are matched parts important?
The notch depends on cancellation. Poor matching leaves more residual signal. Use precision resistors, stable capacitors, or trimming parts.
Can this filter remove 60 Hz hum?
Yes. Set the target frequency to 60 Hz. Use low tolerance parts and proper buffering for deeper hum rejection.
Can this filter remove 50 Hz hum?
Yes. Set the target frequency to 50 Hz. The calculator will solve matching component values for that frequency.
What does the feedback factor mean?
It estimates active feedback used to sharpen the notch. Higher values raise Q, but excessive feedback can cause ringing.
Why does load resistance matter?
A low load can disturb the network. It may shift the notch and reduce attenuation. A buffer helps preserve response.
Is the estimated notch depth exact?
No. It is a practical estimate based on mismatch. Real circuits also depend on layout, source impedance, load impedance, and parasitics.