When would you use a band-reject filter instead of a bandpass filter?

Use a band-reject filter when you need to remove a known interferer from a wideband signal while preserving everything else. Common scenarios: suppressing transmitter harmonics (a 2nd harmonic trap at 2*f_tx), removing a co-site interference source on a ship or aircraft where multiple radios operate simultaneously, eliminating a GPS jammer's signal before the receiver, or notching out a specific frequency in a spectrum analyzer or receiver test setup. A bandpass filter selects what you want; a band-reject filter removes what you do not want. If the interference is narrowband and the desired signals span a wide band, the notch filter is the right choice.

What is the tradeoff between notch depth and bandwidth?

A deeper notch with a given resonator Q requires a narrower rejection bandwidth. This is the fundamental Q-bandwidth relationship: 3 dB notch bandwidth = f_center / Q_loaded. A resonator with Q = 1000 at 1 GHz produces a 3 dB notch bandwidth of 1 MHz. To achieve 40 dB of rejection within that 1 MHz, the resonator must maintain its Q across the entire notch band. If you need deeper rejection over a wider band, you must use higher-order filters (more resonators) or accept higher passband insertion loss. This tradeoff is exactly analogous to the Bode-Fano limit but applied to the rejection band.

RF Components

Band-Reject Filter (Notch Filter)

Q: How deep can a notch filter reject?

A single LC notch or quarter-wave stub can achieve 20-30 dB of rejection at the center frequency. Multi-section notch filters using coupled resonators or cascaded stubs can achieve 40-60 dB of rejection over a narrow bandwidth. YIG-tuned notch filters provide 30-50 dB rejection that is electronically tunable over multi-octave ranges (2-18 GHz). The depth of the notch is limited by the unloaded Q of the resonators and the precision of the tuning. High-Q cavity notch filters for base station applications can achieve over 60 dB rejection with less than 0.5 dB passband insertion loss within 1% bandwidth of the notch center.

/band ree-jekt fil-ter/

A Band-Reject Filter (also called a notch filter or band-stop filter) attenuates signals within a specific frequency band while passing all signals above and below that band with minimal insertion loss. Used to suppress known interferers, remove transmitter harmonics, eliminate co-site interference on multi-radio platforms, and notch spurious products in wideband receiver chains.

Category: RF Components

Notch Depth: 20-60+ dB

Also: BSF, notch filter

Understanding Band-Reject Filters

While a bandpass filter selects what you want, a band-reject filter removes what you do not want. On a naval ship with dozens of antennas operating simultaneously, a radar transmitter's harmonics can land directly on a communications receiver's frequency. A precisely tuned notch filter on the receiver input removes the harmonic without disturbing any other signals. The challenge is achieving deep rejection at the notch frequency with minimal impact on nearby passband frequencies.

Notch Filter Design

Band-Reject Filter (Notch Filter):
A Band-Reject Filter (also called a notch filter or band-stop filter) attenuates signals within a specific frequency band while passing all signals above and below...

Key specifications:
3 dB | -2 GHz | -35 dB | -1.5 dB | -30 GHz | -25 dB

Q factor: Q = f₀/BW_3dB

Notch Filter Technology Comparison

Technology	Frequency	Notch Depth	Passband IL	Tunable?	Application
LC lumped	DC-2 GHz	20-35 dB	0.5-1.5 dB	Varactor	HF/VHF receivers
Stub (microstrip)	1-30 GHz	15-25 dB	0.3-1 dB	No	PCB harmonic traps
Cavity notch	0.3-6 GHz	40-60+ dB	0.2-0.5 dB	Mechanical	Base station co-site
YIG sphere	2-18 GHz	30-50 dB	1-3 dB	Magnetic (wide)	EW, spectrum analyzers
Waveguide iris	8-100 GHz	30-50 dB	0.1-0.3 dB	No	Satellite, radar

Key Equations

Insertion loss:
IL = −20log|S₂₁| dB

Return loss:
RL = −20log|S₁₁| dB

VSWR from Γ:
VSWR = (1+|Γ|)/(1−|Γ|)

Comparison

Band	Range	Wavelength	Application	Standard
Band-Reject Filter (Notch Filter)	1 GHz region	300.0 mm	Primary use	ITU allocation
Adjacent lower	0.9 GHz	333.3 mm	Related band	Shared spectrum
Adjacent upper	1.1 GHz	272.7 mm	Related band	Guard band
Harmonic 2f	2.0 GHz	150.0 mm	Spurious	Filter required
Sub-harmonic	0.5 GHz	600.0 mm	LO option	Mixer design

Common Questions

Frequently Asked Questions

When use a notch filter instead of a bandpass?

When you need to remove a known interferer from a wideband signal while preserving everything else: transmitter harmonic suppression (2nd harmonic trap at 2*f_tx), co-site interference on ships/aircraft, GPS jammer removal, or notching specific frequencies in test setups. A bandpass selects what you want; a notch removes what you do not.

How deep can a notch filter reject?

Single LC or stub: 20-30 dB. Multi-section coupled resonators: 40-60 dB. YIG-tuned: 30-50 dB over multi-octave tuning (2-18 GHz). High-Q cavity notch for base stations: 60+ dB with under 0.5 dB passband IL within 1% of notch center.

What is the depth vs. bandwidth tradeoff?

3 dB notch bandwidth = f_center / Q_loaded. Q=1000 at 1 GHz gives 1 MHz notch bandwidth. Deeper rejection over wider bands requires more resonators (higher order) or accepting more passband loss. This tradeoff parallels the Bode-Fano limit applied to the rejection band.

Related Terms

← Bandpass Sampling Bandwidth →