Resistance-capacitance filter networks with single-component frequency-control

IN RECENT years resistance-capacitance filters employing the Wein bridge, the parallel-T and other circuits have come into widespread use for the frequency control of oscillators and tuned amplifiers. Normally two or more ganged components are used for this purpose although a single component control has been used in several variants of an oscillator circuit described by Johnson.1–3 In certain applications, such as low frequency tuned amplifiers, two or more stages of filtering are desirable in order to achieve adequate selectivity together with quick response to changes in amplitude. The problem of ganging the many variable components then becomes formidable. For example, in an actual case which led to this investigation, a tuned amplifier was required to cover the range from 20 to 60 cps with two stages of filtering. A resistance-capacitance filter was sought for this purpose in which frequency could be controlled over a continuous range of 3 to 1 using a minimum number of variable components. The results of this investigation are reported in the present paper where several new bridge and ladder networks are described in which the balance frequency is controlled by means of a single variable component.