One of the difficulties in the design of higher-order tunable bandpass filters is achieving correct tracking of the variable resistors in the RC networks. The use of switched
capacitor networks can obviate that difficulty, as is shown in this
filter. The filter can be divided roughly into two stages: an oscillator
that controls the electronic switches arid the four phase-shift
networks that provide the filtering proper.
The oscillator, based on a
555, generates a pulsating signal whose frequency is adjustable over a
wide range: the duty factor varies from 1:10 to 100:1. Electronic
switches ESI through ES4 form the variable resistors whose value is
dependent on the frequency of the digital signal. The operation of these
switches is fairly simple. When they are closed, their resistance is
about 60 ; when they are open, it is virtually infinitely high. a
switch is closed for, say, 25% of the time, its average resistance is
therefore 240 . `Varying the open:closed ratio of each switch varies the
equivalent average resistance.
The switching rate of the switches must
be much greater than the highest audio frequency to prevent audible
interference between the audio and the clock signals. The input signal
causes a given direct voltage across CI, so the op amp can be operated
in a quasisym-metric manner, in spite of the single supply voltage. The
direct voltage is removed from the output signal by capacitor C10. The
fourth-order filter in the diagram can be used over the entire audio
range and it has an amplification of about 40, although this depends to
some extent on the clock frequency. The bandwidth depends mainly on the
set frequency. The circuit draws a current of not more than 15 mA.
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