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Bandpass filter - BP/04/N - BP/04/W

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Fixed frequency bandpass filters 1Hz/15kHz tuning range

Customised fixed frequency bandpass filters, finding application in vibration and noise measurement where specific components of a wideband signal spectrum need be highlighted.

Applications include monitoring and quality test procedures relating to fixed speed rotating machinery, enhancing fault diagnosis, or generalising, bandwidth constraint to improve signal/noise or highlight spectral components. Input signal should be steady state or slow varying in the sense that filter transient  response is damped oscillatory and therefore colours the output.

Inputs derived from rotating plant may vary infrequency due to load factors etc... Filter bandwidth fmax- fmin may be extended to compensate, however fmax/fmin>2 will introduce fmin harmonics into the passband.


BP/04 filters are configured as either a stagger tuned triple two pole bandpass for overall bandwidth < 0.3fo(/N), or as a cascaded 7 pole high/low pass Butterworth pair for bandwidth > 0.3fo(/W). Both versions comprise active 2 pole R-C sections but differ conceptually due to variations in tuning requirement.

Passband gain is unity, in the stopband gain rails asymptotically to zero for each type, limited practically by breakthrough. A front  panel mounted switch provides a filter bypass facility (DIR).


For BP/04/N, it is sufficient to specify centre frequency (fo) and Q or % bandwidth. For BP/04/W, fmax and fmin (-3dB corner frequencies) should be referenced.


  • fo, fmax, fmin: centre freq. upper & lower -3dB corner freq's
  • -3dB bandwidth = 1/Q = (fmax- fmin)/fo,applicable to /N
  • fo=    fmaxx fmin