Electronically controllable filter

Details Electronically controllable filter Electronically controllable filter

1977-09-28

1978-10-24

Dahl, Lawrence J.

Amplifiers

Signal feedback

Phase shift means in loop path

330303, H03F 136

Patent

active

041223985

ABSTRACT:
An active filter, capable of being electronically controlled over a 50:1 or greater frequency range, while maintaining a fixed filter characteristic and a constant, nearzero DC offset voltage. The filter utilizes passive element-multipliers, either capacitor-multiplier or resistor-multiplier circuits, as the frequency determining networks, depending upon whether the filter is a low-pass or a high-pass filter. Tuning is accomplished by varying the gain of a variable gain amplifier connected in series with the capacitive or resistive element of the multiplier networks. Also disclosed is an implementation of the variable gain amplifier which permits the filter's cut-off frequency to be controlled digitally. The filter may be designed to have any desired type of filter characteristic. Several filter sections may be connected in cascade to provide any desired number of filter poles.

REFERENCES:
patent: 3702405 (1972-11-01), Zwirn et al.
patent: 3919648 (1975-11-01), Uetrecht
Burr-Brown-Handbook of Operational Amplifier Active R-C Networks, 1966, pp. 2, 43.

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