Notch Filter Calculator

A notch filter, also known as a band-stop or band-reject filter, is an electronic filter that passes most frequencies unaltered but attenuates a very specific, narrow frequency range to very low levels. It is the opposite of a band-pass filter. The "notch" in the frequency response graph gives it its name.

This type of filter is highly selective, meaning it affects only a small band of frequencies. This makes it ideal for removing a single, unwanted interfering frequency from a signal, such as the 50Hz or 60Hz "hum" from AC power lines that can contaminate audio or sensor readings.

Center Frequency ($f_c$): This is the frequency at which the filter provides the maximum attenuation. It is the center of the "notch".

Q Factor (Quality Factor): The Q factor is a dimensionless parameter that describes how narrow the notch is. A higher Q factor corresponds to a narrower and deeper notch, making the filter more selective. A lower Q means a wider notch.

Bandwidth (BW): The range of frequencies that are attenuated. It's related to the center frequency and Q factor by the formula: $BW = f_c / Q$.

Passive Twin-T Notch Formula: For a standard Twin-T circuit, the center frequency is determined by the resistors (R) and capacitors (C):

$$f_c = \frac{1}{2 \cdot \pi \cdot R \cdot C}$$

• Audio Engineering: Removing power-line hum (50/60 Hz) from audio signals, eliminating feedback frequencies in live sound systems, and for creative sound shaping.
• Instrumentation & Control: Filtering out known interference frequencies in sensitive measurement equipment (like ECG/EEG machines).
• Radio Communications: Rejecting strong, interfering signals from nearby transmitters to improve reception of a desired signal.
• Speaker Crossovers: Used to flatten the impedance curve of a speaker driver at its resonant frequency, allowing for a more effective crossover design.