- on Miscellaneous
- Published in issue 7/1989 on page 16
Tuneable Band-Pass Filter
16 One of the difficulties in the design of high- er-order tuneable band-pass filters is achieving correct tracking of the variable resistors in the RC networks. The use of switched capacitor net- works can obviate that difficulty as is shown in this filter. The filter may be divided roughly into two stages: an oscillator that controls the electronic switches and the four phase-shift networks that provide the filtering proper. R3 alca, 1C2 1C1 ne 4,67,7 O 12V BS250 TUNEABLE BAND-PASS FILTER 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. 1:)2 11p R4 C2 R5 1809 ES1 +.4 3. 3 Electronic switches ES1-ES4 form the vari- able 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 0; when they are open, it is virtually infinitely high. If a switch is closed for, s...