Introduction to Filters and Filter Design Software - Practical Analog and Digital Filter Design - page 5

frequency signals. A highpass filter set to a passband edge frequency of 100 Hz
could help to eliminate this distracting signal.
1.1.3 Bandpass Filters
The filter specification for a bandpass filter shown in Figure 1.3 requires a bit
more description. A bandpass filter will pass a band of frequencies while
attenuating frequencies above or below that band. In this case the passband exists
between the lower passband edge frequency f pass1 and the upper passband edge
frequency f pass2 . A bandpass filter has two stopbands. The lower stopband extends
from zero to f stop1 , while the upper stopband extends from f stop2 to infinity (for
analog filters). Within the passband, there is a single passband gain parameter a pass
in decibels. However, individual parameters for the lower stopband gain a stop1
(dB) and the upper stopband gain a stop2 (dB) could be used if necessary.
Figure 1.3 Bandpass filter specification.
A good example for the application of a bandpass filter is the processing of
voice signals. The normal human voice has a frequency content located primarily
in the range of 300-3,000 Hz. Therefore, the frequency response for any system
designed to pass primarily voice signals should contain the input signal to that
frequency range. In this case, f pass1 would be 300 Hz and f pass2 would be 3,000 Hz.
The stopband edge frequencies would be selected by how fast we would want the
signal response to roll off above and below the passband.
1.1.4 Bandstop Filters
The final type of filter to be discussed in this section is the bandstop filter as
shown in Figure 1.4. In this case the band of frequencies being rejected is located
between the two passbands. The stopband exists between the lower stopband edge
frequency f stop1 and the upper stopband edge frequency f stop2 . The bandstop filter