Q = fc/BW. Higher Q means a narrower bandwidth relative to the centre frequency. A Q of 10 means the bandwidth is 1/10th of fc. High-Q filters are very selective.

How do I build a bandstop filter?

Use a series RLC circuit (bandstop at resonance), cascade a high-pass and low-pass filter, or use an active filter with an op-amp. The RLC approach gives the highest Q for passive designs.

What is the centre frequency?

The geometric mean of the lower and upper cutoffs: fc = √(fL × fH). For narrow bandwidths, fc ≈ (fL + fH)/2 (arithmetic mean is close to geometric mean).

Band Stop Filter Calculator

This calculator requires JavaScript. f_c = √(f_L × f_H) | BW = f_H − f_L | Q = f_c/BW

f_c = √(f_L×f_H)

BW = f_H − f_L

Q = f_c/BW

Band Stop — Enter Lower and Upper Cutoff

f_L Lower Cutoff

f_H Upper Cutoff

Enter frequency limits or RLC values

Band Stop Results

Centre fc
—
Hz

Bandwidth

—

Q Factor

—

f_L (−3dB low)

—

f_H (−3dB high)

—

ω_c

—

rad/s

Figure 1: A band stop filter rejects a range of frequencies between f_L and f_H, passing everything outside this band. The centre frequency f_c = √(f_L×f_H) and bandwidth BW = f_H − f_L.

Table of Contents

Fundamentals

What Is a Band Stop Filter?
How It Works

Worked Examples

Removing 50 Hz Mains Hum
Eliminating RF Interference

Deep Dive

Twin-T Notch Filter
Notch Depth and Q Factor

Reference

Frequently Asked Questions
Related Filter Calculators

What Is a Band Stop Filter?

A band stop (notch) filter rejects a range of frequencies between f_L and f_H, passing everything outside this band. It is used to remove specific interference frequencies such as 50/60 Hz mains hum, while preserving the rest of the signal. The Filter Cutoff Frequency Calculator covers the fundamental cutoff frequency calculation that defines every band stop filter.

How It Works

Band Stop Filter: f_c = −3 dB point where output power drops to 50%.
Below f_c: stopband | Above f_c: stopband

The filter response is determined by the component values and circuit topology. First-order filters (single RC or RL) give −20 dB/decade roll-off. Second-order (LC or cascaded) give −40 dB/decade. The RC Filter Calculator shows how different component combinations affect the response.

Worked Example — Removing 50 Hz Mains Hum

Given: R = 10 kΩ, C = 100 nF

f_c = 1/(2π × 10000 × 100×10⁻⁹) = 159.2 Hz

The notch blocks frequencies around the centre.

The RL Filter Calculator handles the individual stages.

Worked Example — Eliminating RF Interference

Given: L = 10 mH, C = 1 µF

f_c = 1/(2π√(0.01 × 10⁻⁶)) = 1592 Hz

Twin-T Notch Filter

A twin-T network using resistors and capacitors creates a passive notch without inductors. The LC Filter Calculator handles the complementary filter type.

Notch Depth and Q Factor

Higher Q means the filter is more selective but also more sensitive to component tolerances. For Q > 10, active filters are generally required. The Low Pass Filter Calculator covers the active version with op-amp gain.

RLC Circuit Calculator: The RLC Circuit Calculator analyses the impedance null at resonance that creates the notch rejection.

Frequently Asked Questions

What is the −3 dB point?

−3 dB is where the output power is half the input power (voltage is 70.7% of input). This is the universally agreed definition of cutoff frequency for all filter types.

Can I use this for audio?

Yes. Audio frequencies range from 20 Hz to 20 kHz. Choose component values that place f_c within this range for audio filtering applications.

What order filter do I need?

First-order (−20 dB/dec) is adequate for gentle roll-off. For sharper transitions, use second-order (−40 dB/dec) or higher. The Butterworth Filter Calculator designs multi-order responses.

Does the filter introduce delay?

Yes. All filters introduce phase shift and group delay. First-order filters have constant group delay (linear phase approximation). Higher-order filters can have significant group delay variation near fc.

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Last updated: March 2026