What is a Notch Filter? Frequency Rejection Tool • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is a Notch Filter? Frequency Rejection Tool • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is a Notch Filter? Frequency Rejection Tool

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Understanding Notch Filters

Definition: What is a Notch Filter?

Notch filter (also called band-stop filter, band-reject filter, or frequency trap) is a frequency-selective signal processing element that strongly attenuates vibration components within a narrow frequency band while allowing all frequencies outside that band to pass essentially unchanged. A notch filter is the opposite of a band-pass filter: instead of passing a band and blocking everything else, it blocks a specific band and passes everything else.

Notch filters are used in vibration analysis to remove dominant interference (60 Hz electrical noise), eliminate overwhelming vibration components (very high 1× unbalance masking other signals), or suppress resonances that obscure diagnostic information. They enable “seeing around” dominant frequencies to reveal weaker but diagnostically important components.

Filter Characteristics

Center (Notch) Frequency

  • Frequency of maximum attenuation
  • The frequency being “notched out”
  • Tuned to specific interference or unwanted frequency
  • Attenuation typically 40-60 dB at center

Notch Bandwidth

  • Narrow Notch: Rejects very selective frequency range (high Q)
  • Wide Notch: Rejects broader frequency band (low Q)
  • Q Factor: Center Frequency / Bandwidth
  • Typical: Q = 10-50 for vibration applications

Attenuation Depth

  • How much the notch frequency is reduced
  • Typically 40-60 dB (100-1000× reduction)
  • Higher order filters provide deeper notches
  • Adjacent frequencies minimally affected

Common Applications

1. Electrical Interference Removal

Eliminating power line noise:

  • 60 Hz Notch: Removes 60 Hz electrical pickup in North America
  • 50 Hz Notch: Removes 50 Hz interference in Europe/Asia
  • Harmonics: Additional notches at 120/180/240 Hz or 100/150/200 Hz
  • Benefit: Cleaner spectrum revealing mechanical vibration
  • Caution: Don’t use if 2× line frequency (120/100 Hz) has diagnostic value

2. Dominant Component Suppression

  • Severe Unbalance: Notch out overwhelming 1× to see other components
  • High Gear Mesh: Remove dominant gear mesh to reveal bearing frequencies
  • Strong Resonance: Suppress structural resonance to see excitation
  • Purpose: Reveal masked diagnostic information

3. Sensor Resonance Elimination

  • Remove sensor mounting resonance artifacts
  • Notch at mounting resonance frequency (varies with mounting method)
  • Ensures measurements represent machine, not sensor

4. Avoiding Aliasing Artifacts

  • Notch out specific high frequencies before downsampling
  • Prevents aliasing of known strong components
  • Complements anti-aliasing low-pass filter

Design Considerations

Notch Width Selection

Narrow Notch (High Q)

  • Advantage: Surgical removal of single frequency, minimal effect on adjacent
  • Disadvantage: Frequency must be precisely known and stable
  • Example: 60.0 Hz ± 0.5 Hz notch for electrical interference

Wide Notch (Low Q)

  • Advantage: Captures frequency variations, less critical tuning
  • Disadvantage: May affect frequencies you want to keep
  • Example: 1× ± 5 Hz to remove unbalance that varies with speed fluctuations

Depth vs. Width Trade-off

  • Deeper notches (> 60 dB) often require wider bandwidth
  • Very narrow notches may not achieve deep attenuation
  • Optimize based on application requirements

Advantages and Limitations

Advantages

  • Removes dominant interfering frequencies
  • Reveals masked diagnostic components
  • Improves dynamic range usage
  • Allows focusing on weaker but important signals

Limitations and Cautions

  • Removes Information: Notched frequency content lost permanently
  • Can Hide Problems: If notched frequency has diagnostic value, problem missed
  • Phase Distortion: Notch filters can significantly affect phase near notch frequency
  • Ringing: Sharp notches can create time-domain artifacts
  • Use Cautiously: Should complement, not replace, unfiltered analysis

Best Practices

When to Use Notch Filters

  • Known interference (electrical noise) obscuring measurements
  • Dominant component (severe unbalance) preventing dynamic range utilization
  • After unfiltered analysis confirms notched frequency not diagnostic
  • To reveal weak signals for detailed examination

When NOT to Use

  • Routine screening measurements (use unfiltered for general diagnostics)
  • When notched frequency has diagnostic value
  • Without first understanding full unfiltered spectrum
  • As substitute for fixing actual interference source

Documentation

  • Always document when notch filter used
  • Record notch frequency and bandwidth
  • Maintain unfiltered data for reference
  • Note reason for notch filtering

Implementation

Hardware Notch Filters

  • Fixed frequency (typically 50 or 60 Hz)
  • Switched in/out as needed
  • Analog circuit in instrument
  • Real-time operation

Software Notch Filters

  • Applied to digitized data
  • Adjustable center frequency and bandwidth
  • Can test different notch parameters
  • Non-destructive (original data preserved)

Notch filters are specialized signal processing tools that selectively remove narrow frequency bands from vibration signals. While powerful for eliminating interference and revealing masked components, notch filters must be used judiciously with full understanding of what information is being discarded, ensuring that the notched frequencies don’t contain important diagnostic content.

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