Bandstop Filter Block Symbol
Definition: The Bandstop Filter Block symbol represents a two-port frequency-selective network that attenuates signals within a defined stop band (between lower cutoff frequency f_L and upper cutoff frequency f_H) and passes signals outside that band with low attenuation, drawn in circuit schematics as a rectangular function block labelled 'BSF' or 'NOTCH' with an In pin and an Out pin, consistent with IEC 60617-09 analogue element block conventions and ANSI/IEEE 315-1975 filter block representations.
Also known as: BSF, notch filter, band reject filter, band elimination filter, band stop filter, trap filter, notch, rejection filter.
What the Bandstop Filter Block symbol means
The Bandstop Filter Block symbol in a circuit diagram represents a frequency-selective element that blocks or strongly attenuates signals within a specific frequency band — the stop band — while passing all other frequencies above and below the stop band with minimal attenuation. The width of the stop band and the depth of rejection are the critical performance parameters annotated on or near the block symbol.
Bandstop filters appear in circuit schematics wherever a specific interfering frequency or frequency band must be suppressed without affecting the rest of the signal spectrum. Applications include mains frequency (50/60 Hz) hum rejection in audio circuits, interference rejection in radio receivers, harmonic suppression in transmitters, and anti-alias filtering. A bandstop filter with a very narrow stop band is called a notch filter; its schematic block is often labelled 'NOTCH' with the specific rejection frequency annotated.
How to identify the Bandstop Filter Block symbol
The Bandstop Filter Block symbol is a rectangle labelled 'BSF', 'NOTCH', or 'BAND REJECT FILTER', annotated with the rejection frequency or stop band range (e.g., '50 Hz notch', '60 Hz notch', '10.7 MHz reject'). The In pin enters the left side and the Out pin exits the right side. Some representations include a frequency response curve sketch inside the block showing a notch or U-shaped dip at the stop band frequencies. The symbol is the visual complement of the BPF block — where BPF shows a bell curve (passband), the BSF shows a notch or inverted bell (stop band).
Function in a circuit
A bandstop (notch) filter attenuates signals at or near its rejection frequency f₀ = √(f_L × f_H) while passing all other frequencies. The depth of rejection (notch depth) may be 20–80 dB depending on the filter implementation. For a single-notch (twin-T) RC filter at 60 Hz, rejection can exceed 40 dB at the notch frequency. For narrow-band RF notch filters, rejection of 60–80 dB is achievable using cavity resonators or helical resonators. The quality factor Q = f₀ / BW determines notch sharpness — a high Q yields deep, narrow rejection; a low Q yields broad, shallow rejection. Bandstop filters may be passive (LC, RC) or active (op-amp with feedback network).
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-09:1993 (Graphical symbols for diagrams — analogue and signal processing elements) defines filter block symbols as rectangular blocks with frequency annotation. A bandstop filter follows the same block convention as the BPF, distinguished by the 'BSF', 'NOTCH', or 'BE' (band elimination) label. IEC 60401-4 defines filter terminology including 'band elimination filter'. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 Section 16 defines filter function blocks as rectangles with type annotation. The bandstop filter is labelled 'BSF' or 'NOTCH FILTER'. IEEE 100 (IEEE Standard Dictionary) defines band-reject and notch filter terms. Both IEC and ANSI use identical rectangular block conventions for all filter types. |
| Key difference | IEC and ANSI/IEEE use identical rectangular blocks for bandstop filter symbols. IEC uses the term 'band elimination filter' (BE) while ANSI/IEEE uses 'bandstop' or 'band-reject'. Both are represented by the same block symbol with different labels. No glyph difference exists. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Notch frequency f₀: audio range 50/60 Hz (mains hum), 100/120 Hz (second harmonic), audio band 20 Hz–20 kHz; RF range: 455 kHz, 10.7 MHz, VHF/UHF to GHz; notch depth: 20–80 dB; stop band: narrow (<1% of f₀ for notch) to wide (octave); insertion loss in passband: <1 dB; Q factor: 1 (broadband) to 10,000 (cavity notch); impedance: 50 Ω (RF), high-Z (audio).
Where the Bandstop Filter Block symbol is used
- Power line hum rejection in audio amplifiers and medical instrumentation: 50 Hz or 60 Hz notch filters removing mains interference from low-level signals
- RF transmitter harmonic suppression: notch at second or third harmonic frequency (2f₀, 3f₀) to meet spurious emission limits
- FM/AM receiver image rejection: notch filter at the image frequency to prevent heterodyne interference in superheterodyne receivers
- ECG and EEG medical signal processing: narrow 50/60 Hz notch to eliminate electrode pickup of mains interference without affecting the biological signal
- Guitar effect pedals and audio processors: notch or parametric equaliser for feedback suppression and tone shaping
- Cable TV and satellite distribution: bandstop filter at interference frequencies (e.g., amateur radio bands, cellular) to prevent ingress noise
- Anti-alias and reconstruction filtering in data acquisition: bandstop notch at the sampling frequency to reject aliased components
Example
In a medical ECG amplifier schematic, a 60 Hz Bandstop (Notch) Filter Block appears in the signal chain between the instrumentation amplifier and the ADC. Annotated '60 Hz, 40 dB rejection', the BSF symbol's In pin receives the amplified ECG signal and Out delivers the filtered signal. The notch eliminates power-line interference while leaving the 0.05–100 Hz ECG signal spectrum intact, enabling accurate cardiac waveform acquisition.
Key facts
- A bandstop filter (BSF, notch filter) attenuates signals within its stop band (f_L to f_H) while passing all other frequencies; it is the frequency-domain complement of a bandpass filter.
- The schematic symbol has two pins: In (left) and Out (right); the block is labelled 'BSF', 'NOTCH', or 'BAND REJECT' and annotated with the rejection frequency (f₀) and stop-band bandwidth.
- A notch filter is a bandstop filter with a very high Q (narrow stop band): f₀ / BW >> 1. Common notch frequencies are 50 Hz and 60 Hz (mains hum) in audio/biomedical circuits and specific RF frequencies in communications.
- The twin-T RC notch circuit is a classic passive implementation providing one null at f₀ = 1/(2πRC); op-amp feedback can sharpen the notch and adjust depth independently.
- Notch depth (rejection at f₀) ranges from 20 dB (simple RC circuits) to 80 dB (cavity resonators and high-Q LC traps); for most interference rejection applications, 40 dB or more is needed.
- The Wien bridge oscillator and twin-T oscillator both use notch filter networks as the frequency-determining feedback element at the oscillation frequency.
- In RF transmitter circuits, a bandstop filter (trap) at the second harmonic (2×f_transmit) suppresses harmonic radiation to comply with regulatory spurious emissions limits (e.g., FCC Part 97, ITU-R).
- The IEC term for a bandstop filter is 'band elimination filter'; ANSI/IEEE uses 'bandstop' or 'band-reject filter'; in telecommunications, 'notch filter' is used when the stop band is very narrow.
Frequently asked questions
What does the Bandstop Filter symbol look like in a circuit diagram?
The Bandstop Filter symbol is a rectangle labelled 'BSF', 'NOTCH', or 'BAND REJECT FILTER', annotated with the rejection frequency or stop band (e.g., '60 Hz notch', '10.7 MHz reject'). The In pin enters the left side and the Out pin exits the right side. Some representations show an inverted-bell or U-shaped notch frequency response sketch inside the block.
What does the bandstop filter symbol mean in a schematic?
The bandstop filter symbol means that signals at or near the marked rejection frequency are attenuated at that point in the circuit, while signals at all other frequencies pass through with minimal loss. The symbol indicates frequency-selective interference suppression rather than simple gain reduction.
What is the difference between a bandstop filter and a notch filter?
A notch filter is a bandstop filter with a very narrow stop band — specifically, a stop band that is much narrower than the centre frequency (high Q). The terms are often used interchangeably in schematics. A bandstop filter may have a wider stop band covering a frequency range, while a notch filter is used when very selective rejection of a single frequency (like 60 Hz or 10.7 MHz) is needed.
What is the difference between a bandpass filter and a bandstop filter?
A bandpass filter (BPF) passes frequencies within the passband and attenuates all others. A bandstop filter (BSF) attenuates frequencies within the stop band and passes all others — they have complementary (inverse) frequency responses. In schematics, BPF is labelled 'BPF' and shows a bell-curve response; BSF is labelled 'BSF' or 'NOTCH' and shows a notch response.
What standard defines the bandstop filter symbol?
The bandstop filter block symbol is defined in IEC 60617-09 (analogue and signal processing elements, rectangular block convention) and ANSI/IEEE 315-1975 Section 16 (frequency-selective components). IEC terminology uses 'band elimination filter'; ANSI/IEEE uses 'bandstop' or 'band-reject'. Both use identical rectangular block representations.
How deep is the notch in a typical bandstop filter?
Notch depth ranges from about 20 dB (simple passive twin-T RC circuit) to over 80 dB (high-Q LC cavity resonator or active op-amp notch with precise component matching). For mains hum rejection in audio, 40–60 dB notch depth is typically sufficient. RF interference rejection for communications often requires 60–80 dB to meet regulatory spurious emission limits.
What are common applications for a 60 Hz notch filter?
A 60 Hz notch filter (bandstop at 60 Hz) is used in audio amplifiers, medical electrodes (ECG, EEG, EMG), and instrumentation amplifiers to suppress power-line interference picked up by electrode cables and circuit wiring. The filter attenuates 60 Hz (or 50 Hz in 50 Hz regions) by 40–60 dB while leaving the signal band (0.05 Hz–20 kHz for audio, 0.05–100 Hz for ECG) unaffected.
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