Notch Filter
Understanding Notch Filters
Notch filters are the opposite of bandpass filters: they remove energy at a specific frequency while passing everything else. They are particularly valuable when a known interferer is corrupting a wideband signal. Rather than narrowing the receiver bandwidth (which would lose wanted information), a notch filter surgically removes only the interference.
Notch Filter Applications
- Interference suppression: Remove a known interfering signal from a wide receiver band.
- Harmonic rejection: Notch out a specific harmonic frequency.
- LO leakage: Remove residual LO signal after mixing.
- Cavity duplexer: Notch filters in the transmit path prevent desensitizing the receiver.
Types
- LC notch: Simple series or parallel LC resonator tuned to the rejection frequency.
- Stub filter: Quarter-wave open or shorted stubs. Used at microwave frequencies.
- Cavity notch: High-Q cavity resonator coupled to the transmission line. Very deep notch (40-60 dB) with minimal passband effect.
- YIG notch: Magnetically tunable notch filter using a YIG sphere.
Rejection depth: 20-60+ dB
3 dB bandwidth: Q = f_notch / BW_3dB
Series LC notch: Z_min at resonance
Parallel LC notch: Z_max at resonance
Quarter-wave stub notch:
stub length = lambda/4 at rejection frequency
Open stub: short circuit at stub end
Short stub: open circuit at stub end
Frequently Asked Questions
What is a notch filter?
A notch filter rejects signals at a specific frequency while passing all other frequencies. It is used to suppress known interferers without affecting wanted signals. Notch filters are the frequency-domain equivalent of surgically removing a single unwanted component.
How narrow can a notch filter be?
The notch bandwidth depends on the resonator Q. Cavity notch filters can achieve rejection bandwidths of less than 0.1% of center frequency with 40+ dB depth. LC notch filters are typically wider. YIG notch filters offer narrow bandwidth with magnetic tunability.
What is the difference between a notch filter and a bandpass filter?
They are complementary. A bandpass filter passes only a narrow selected band and rejects everything else. A notch filter rejects only a narrow band and passes everything else. A notch filter removes interference; a bandpass filter selects the desired signal.