Page 148 - The Master Handbook Of Acoustics

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ANALOG AND DIGITAL SIGNAL PROCESSING

High-pass filter Low-pass filter

A

B

C

FIGURE 6-5

High-pass and low-pass filters of the simplest form. The filters in (C) will have sharper
cut-off than the others.

Adjustable filters can be readily shifted to any frequency within their
design band. One type is the constant bandwidth filter which offers the
same bandwidth at any frequency. For example, a spectrum analyzer may
have a 5-Hz bandwidth whether it is tuned to 100 Hz or 10,000 Hz, or any
other frequency within its operating band. An even more widely used
adjustable filter offers a pass band-width that is a constant percentage of
the frequency to which it is tuned. The ⁄3-octave filter is such a device. If
1
it is tuned to 125 Hz the ⁄3-octave bandwidth is 112 to 141 Hz. If it is tuned
1
to 8,000 Hz the ⁄3-octave bandwidth is 7,079 Hz to 8,913 Hz. The band-
1
width is about 23% of the frequency to which it is tuned in either case.
Active Filters
Active filters depend on integrated circuits for their operation. An
integrated circuit can have many hundreds of components in a small
enclosure. Their fabrication depends on growing transistors and