Notch Frequency

f = 1 / (2π√(LC))

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Result
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Formula

f = 1 / (2π√(LC))

Description

A notch (band-stop or band-reject) filter blocks a narrow range of frequencies centered at the notch frequency while passing all others. The center frequency is determined by the LC resonance, using the same formula as the resonant frequency calculator. At this frequency, the series LC branch has zero impedance (short circuit) or the parallel LC branch has infinite impedance (open circuit), depending on the topology. Notch filters are used to remove specific interference frequencies, such as 50/60 Hz power line hum in audio systems or specific carrier frequencies in communication systems.

Variables

  • f — Notch center frequency (Hz)
  • L — Inductance (H)
  • C — Capacitance (F)

Practical Notes

The Twin-T RC notch filter is a popular active implementation that does not require an inductor. It uses three resistors and three capacitors arranged in two T-networks. The depth of the notch depends on component matching: 1% tolerance components can achieve 30-40 dB rejection. Digital notch filters (IIR or adaptive) can achieve 60+ dB rejection and can track varying interference frequencies.

Related Formulas

RC Low-Pass Filter RC High-Pass Filter Bandpass Bandwidth Filter Rolloff Rate Butterworth Q Factor Sallen-Key Cutoff Frequency

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