5G communication testing, wireless terminal automated testing
Aerospace communication systems
Precision measurement equipment, radio frequency signal control of radar transmitters
Communication and navigation systems
Optical communication module-related testing
Interconnection of wireless communication base stations
A band stop filter passes frequencies outside a defined stopband and attenuates those inside it. When the stopband is narrow it is called a notch filter; when it is wide it is called a band reject filter. The response is essentially the inverse of a bandpass filter, and it is widely used to remove 50/60 Hz hum, reject TV-band interference, suppress RF carriers, and clean up power-line communication noise.
Formally, a band stop filter is a two-port network whose magnitude response |H(f)| is near unity for frequencies below the lower cutoff fL and above the upper cutoff fH, while attenuating frequencies between them. Three parameters describe the response:
A common special case is the notch filter, where BW is very narrow and the filter removes a single interfering tone (e.g., 50 Hz mains hum or a CW radar signal).
Frequencies between fL and fH fall into the stopband and are attenuated, while everything outside that band passes through. The deepest notch occurs at the center frequency f0.
Band stop filters are usually built using one of three architectural approaches:
| Aspect | Notch Filter | Band Reject Filter |
|---|---|---|
| Stopband Width | Very narrow (< 5% of f0) | Wide (10% – 90% of f0) |
| Typical Use | Single-tone interference removal | Reject a whole sub-band |
| Implementation | Twin-T, lattice, FIR comb | LC ladder, cavity, ceramic |
| Example | 60 Hz hum removal | Reject 88 – 108 MHz FM band |
A parallel LC tank placed in series with the signal path creates a notch at f0 = 1/(2π√LC). Simple and rugged.
Classic op-amp topology providing deep nulls (>40 dB) at a single frequency with good Q control.
Selective notch with independent Q adjustment; popular in audio feedback suppression.
Microwave cavities tuned to reject a specific carrier or radar signal with very high Q.
An open or shorted transmission-line stub presents an open circuit at its quarter-wave frequency.
Piezoelectric resonators with extremely high Q for very narrow, deep rejection.
Compact ceramic resonators for cellular and IoT band rejection (e.g., LTE band 13).
Software-implemented notch with adaptive frequency tracking; ideal for biomedical and audio DSP.
| Parameter | Description |
|---|---|
| Center Frequency (f0) | Midpoint of the rejection band |
| Notch Depth | Maximum attenuation within the stopband (40–60 dB typical) |
| −3 dB Bandwidth | Width of the stopband at the −3 dB attenuation points |
| Quality Factor (Q) | f0 / BW; controls selectivity and ringing |
| Passband Ripple | Allowed variation outside the stopband |
| Stopband Rejection | Minimum attenuation over the entire stopband |
| Insertion Loss | Loss in the passband (ideally 0 dB) |
| Group Delay | Time delay; high Q introduces ringing near f0 |
The most common audio application: a narrow notch at 50 Hz or 60 Hz removes mains interference from biomedical, instrumentation, and live-audio signals without affecting surrounding audio content.
PLC systems inject data onto mains wiring. A band stop filter isolates the data band (often 30 – 500 kHz) so it doesn't disturb audio systems or other equipment sharing the same line.
Live-sound consoles deploy automatic notch filters that detect and cancel feedback frequencies in real time, often reducing 5 – 10 dB of ringing without affecting program material.
After the digital TV transition, many countries freed the 700 MHz band for wireless microphones. Notch filters suppress legacy TV carriers and strong radar pulses in shared spectrum.
High Q gives deep, narrow notches but causes ringing on transients. Use the lowest Q that achieves your required stopband depth.
LC and microstrip notch filters are designed for a specific source and load impedance (often 50 Ω or 75 Ω). Mismatches shift f0 and reduce notch depth.
LC components drift with temperature. For stable notches, use NP0/C0G capacitors or digital implementations that retune themselves.
Two cascaded notch filters can produce >60 dB rejection and a wider flat-bottom stopband without raising Q too high.
Combine a band stop filter with a bandpass or lowpass filter when you need both rejection and bandwidth limitation. The pre-filter protects the notch from overload.
Tools like LTspice, QUCS, RFsim99, or Python SciPy predict insertion loss, group delay, and tuning sensitivity in minutes. Always simulate before committing to hardware.
Network analyzer or swept sine measurements are essential. A notch that "looks" deep in simulation can be shallower due to parasitics.
When the interference frequency drifts, use an adaptive notch filter driven by an LMS or RLS algorithm that tracks the unwanted tone automatically.
| Aspect | Band Stop | Band Pass |
|---|---|---|
| Response Shape | Inverse "U" — passes outside, blocks inside | "U" — passes inside, blocks outside |
| Common Use | Reject interference | Select desired signal |
| Twin-T equivalent | Yes (notch configuration) | Yes (bandpass configuration) |
| Combined | Cascading a band stop with a band pass yields highly selective filtering for crowded spectrum | |
A band stop filter is a circuit or algorithm that attenuates a specific band of frequencies while passing all others. It is the inverse of a bandpass filter and is widely used to remove interference.
A notch filter is a band stop filter with a very narrow stopband. It removes a single interfering tone, such as 50 Hz or 60 Hz mains hum, without disturbing nearby frequencies.
They are functionally the same — both attenuate a band of frequencies. "Band reject" is often used for wider stopbands, while "notch" refers to very narrow ones.
Active twin-T notch filters can reach 40 – 50 dB of attenuation. Cascading two stages or using mechanical resonators (cavity, crystal) can exceed 60 – 80 dB.
Yes. Digital IIR notch filters with LMS or RLS adaptation can automatically track drifting interference frequencies, making them useful for motor whine, variable-speed drives, and biomedical applications.
High-tech Enterprise, Feifeng Talent
Chief Engineer Wang graduated with a master's degree in high-power microwave from the Institute of Electronics, University of Chinese Academy of Sciences.
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