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Band-Rejection Filter Design
mid-frequency:
passband gain:
f m 1
2RC
A 0
1
rejection quality:
Q 1
3
To calculate the individual component values, establish the following design procedure:
1) Define f m and C and calculate R with:
R 1
2f m C
2) Specify Q and determine α via:
3Q 1
3) Specify A 0 and determine β via:
A 0·3Q
4) Define R 2 and calculate R 3 and R 4 with:
R 3
R 2
and
R 4
R 2
In comparison to the twin-T circuit, the Wien-Robinson filter allows modification of the
passband gain, A 0 , without affecting the quality factor, Q.
If f m is not completely suppressed due to component tolerances of R and C, a fine-tuning
of the resistor 2R 2 is required.
Figure 16–41 shows a comparison between the filter response of a passive band-rejection
filter with Q = 0.25, and an active second-order filter with Q = 1, and Q = 10.
16-40