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Adjustable notch filter

2015-01-15 00:25  
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In applications where the rejected signal might deviate slightly from the null on the notch network, it is advantageous to lower the Q of the network. This insures some rejection over a wider range of input frequencies. The figure shows a circuit where the Q may be varied from 0.3 to 50. A fraction of the output is fed back to R3 and C3 by a second voltage follower, and the notch Q is dependent on the amount of signal fed back.
Adjustable notch filter

A second follower is necessary to drive the twin `T` from a low-resistance source so that the notch frequency and depth will not change with the potentiometer setting.

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