interferes with the station you wish to listen to. Figure 11
shows a twin T band stop filter schematic and Figure 12
shows the frequency response curve for the twin T.
The BRF frequency response is like an inverted version
of the response for the band pass filter. The center
frequency for the twin T is fCENTER = 1/(2ΠRC) and a
bandwidth of BW = 2/(RC). The twin T is actually a high
pass filter (C2, C3, and R3) in parallel with a low pass filter
(R1, R2, and C1) where the notch is created by the
combined parallel resonances of these two filters which
produces a high impedance to the signal in the stop band.
Sometimes the BRF is used to eliminate power line
frequencies in audio systems.
So far, we have looked at passive filters which have no
active elements (transistors or amplifiers). By adding an
active element such as an operational amplifier (op-amp),
All Pass Filters (APF) provide equal gains for all input
signals over a range of frequencies (flat frequency
response) by phase shifting the various frequencies in a
predictable manner. The APF can be used as a delay
equalizer or phase corrector. Figure 14 shows an active
APF and the phase delay produced by this filter. APFs are
used in audio system reverberation units and speaker
crossovers.
I have left out a lot of filters such as Butterworth,
Chebyshev, Bessel, Pi, L, Surface Acoustic Wave
(SAW), multi-pole, Sallen Key, etc. Also, I have
not covered a lot of filter design in the form of
Laplace transforms, transfer functions, and
impulse response, which are the subjects of
college engineering classes. See Q&A
SIDELINES for online filter calculator websites.
NV
■ FIGURE 11.
■ FIGURE 12.
■ FIGURE 13.
■ FIGURE 14.
■ FIGURE 15.
February 2016 13
